Wednesday, July 10, 2019

は vs. が

WIP : this article is incomplete and might change in the unforeseeable future.

In this article, I'll explain the difference between wa は and ga が, the two most confusing particles in all Japanese.

Functions

The wa は particle marks the topic and the ga が particle marks the subject.

uso de aru
嘘である
[That] is a lie.

Well, it's not a complete lie, but it's like two half-truths that produce one quarter of the truth together.

Since these half-truths get repeated around a lot, a beginner might think:

I don't know what a "topic" is, and maybe I don't even know what a "subject" is, but once I figured those out, it'll be that easy.

It won't be that easy.

The whole truth is that wa は and ga が have multiple functions besides marking the topic and subject. They are:

Topic-marker wa は.
Contrastive wa は.
Adverbial wa は.
Neutral-description ga が.
Exhaustive-listing ga が.
Genitive-case marker ga が.
Conjunctive ga が.

In this article I'm going to show how each one of these work, so you're able to tell when to use wa は and when to use ga が, or why one is used over the other in a Japanese text.

This is going to require learning a lot about grammar, so let's go step by step.

Topic vs. Subject

The subject and object are arguments of a verb. Whenever you have a verb, you'll have a subject somehow related to that verb. If the verb is transitive, you'll have an object too.

The cat slept.
- The cat - subject.
- To sleep - verb.
I ate a banana.
- I - subject.
- To eat - verb.
- A banana - object.

The sentences above are in active voice. This means the subject is also the agent doing the action, and the direct object is the patient receiving the action.

In the passive voice, the subject is the patient receiving the action instead.

The banana was eaten by me.
- The banana - subject, patient.
- To be eaten - verb.
- Me - agent.

In Japanese, the ga が particle marks the subject, the wo を particle marks the direct object, and the ni に particle marks the agent in passive voice.

neko ga neta
猫が寝た
The cat slept.
- neko ga - subject, agent.
- neta - verb in past form.
watashi ga banana wo tabeta
私がバナナを食べた
I ate a banana.
- watashi ga - subject, agent.
- banana wo - direct object.
- tabeta - verb in past form.
banana ga watashi ni taberareta
バナナが私に食べられた
The banana was eaten by me.
- banana ga - subject, patient.
- watashi ni - agent.
- taberareta - verb in passive past form.

Furthermore, a copula can be used to connect the subject to a subject complement, which describes it.

tsuki ga kirei desu
月が綺麗です
The moon is pretty.
- tsuki ga - subject.
- kirei - complement.
- desu - polite copula.

The desu です copula is polite so it's always there to sound polite. On the other hand, the da だ copula is plain: it doesn't have the added function of making things polite. Consequently, it gets omitted often.

tsuki ga kirei da
月が綺麗だ
(same meaning.)
tsuki ga kirei
月が綺麗
(same meaning.)

The topic and focus are concepts of information structure. Roughly speaking, the focus of a sentence is the new information, while the background is the old information.

Most of the time you say something new, you say something new about something old. That is, if I tell you something you don't know about yet, it's probably about something you already know about.

The Earth is flat.

For example, if I tell you that "the Earth is flat," this is a new information regarding "the Earth." You already know the Earth, right? So that's background. The fact that the Earth "is flat," however, is new information for you, so that's the focus.

Alternatively, you could call "saying new stuff about old stuff" the act of "saying a comment regarding a topic."

Chikyuu wa hiratai
地球は平たい
The Earth: flat is.
The Earth is flat.
- chikyuu wa - topic.
- hiratai - comment.

The problem is, "saying new stuff about old stuff" is a pretty vague concept. All we can be sure about is that the comment contains information that's somehow connected to the topic.

The comment can expand what we know about the topic. It can build on top of the information we already have available. It can be a statement that's only true and only applies to the topic. It can be anything.

We have no idea how exactly the new and old information will be related.

For example, if the comment is an action, it follows that the topic is somehow related to that action. The topic could be the agent doing the action, or the patient receiving the action. In which case, the topic is also be the subject.

Chikyuu wa sonzai shinai
地球は存在しない
The Earth: exist does not.
The Earth doesn't exist.
- Chikyuu wa - topic, agent, subject.
- sonzai shinai - verb in negative form.

A common misconception beginners have is that, because ga が marks the subject, the subject must then be marked by ga が. In other words, if you don't have a ga が, you don't have a subject. This is incorrect.

In Japanese, it's more important to mark the topic than it's to mark the subject, so if the same thing is both the topic and the subject, it only gets marked as the topic.

Since most of the time you say something new, you say something new about something old, most of the time the subject is also the topic, so you'll end up marking it with wa は instead of with ga が.

neko wa neta
猫は寝た
The cat slept.
- "What the cat did" is new information regarding the cat.
watashi wa banana wo tabeta
私はバナナを食べた
I ate the banana.
- "What I ate" is new information regarding me.

The trick is, then, to know when you have a subject that's only the subject and not the topic. To know when your subject isn't what your sentence is about.

Interrogative Pronouns

The most clear-cut situation where you can't use wa は happens with questions. Specifically, when a question has a subject that is an interrogative pronoun.

The reason for this is quite simple: an interrogative pronoun represents something you don't know about yet—What? Which? Who? When? Where? Why? How?—if you don't know about it, it isn't "old information," so it isn't background, so it's focus. And if it's focus, it can't be the topic.

To elaborate, let's ask a series of questions. Below, the focus is in bold, and everything that's not in bold is the background.

Who ate the banana?
- I ate the banana.
You ate what?
- I ate the banana.
What happened?
- I ate the banana.

When people ask questions, they're always seeking new information (focus) about something that they already know about (background).

There's no way to formulate a question about nothing. Even if you said just "what" you'd be forcing the hearer to figure out about what you're asking about. Fundamentally, it's because you're aware of only part of something that you seek to know the part you don't know about yet.

In the first question above, the questioner knows "ate the banana" happened, but doesn't know the culprit behind the act. He doesn't know the agent, the subject.

In the second question, he knows "you ate," but now what you ate. He doesn't know the object.

In the last, he knows something "happened." Something is always happening, so this is as abstract as we can get.

The answers then have subject focus, object focus, and sentence focus respectively.

Since the topic is part of the background, and the background is the opposite of the focus, you can't mark as the topic something that's the focus. In other words, you can't say this:

*dare wa banana wo tabeta?
~~誰はバナナを食べた？~~
(if I knew "who" I wouldn't be asking.)

If you said that, dare would be the background, and banana wo tabeta would be the focus, which is the complete opposite of the reality: banana wo tabeta is the background, it's the part we know about, while dare is the focus, it's the part we don't know about yet.

{banana wo tabeta} no wa dare?
バナナを食べたのは誰？
The one [who] {ate the banana} is who?
- {banana wo tabeta} no wa - topic, background.
- dare - comment, focus.

Above, we have the correct focus-background, but that's an unnatural way of saying it. The normal way of saying it would be:

dare ga banana wo tabeta?
誰がバナナを食べた？
Who ate the banana?

However, here we've run into a technical problem.

I said before that in Japanese it's very important to mark the topic. As we've seen, we do have a topic in our question above: the fact that someone "ate the banana." And yet, we aren't marking our topics as topics. Why is that?

This is a second function of the ga が particle: exhaustive listing. Effectively, it marks the focus of the sentence, and consequently everything else is the background.

nani ga nusumareta?
何が盗まれた？
What was stolen?
- nani ga - focus, subject, patient.
- nusumareta - background, verb in passive past form.
ore no saifu ga nusumareta
俺の財布が盗まれた
My wallet was stolen.
- ore no saifu ga - focus, subject, patient.

Its name, "exhaustive listing," comes from the fact it (sometimes) exhaustively lists the answer for a question. For example, above I said "my wallet was stolen," which means nothing else was stolen. If knew something else was stolen, I'd have said so. I'd have listed everything, exhaustively.

The Maxim of Quantity. ^{(Grice, 1975)}
1. Make your contribution as informative as required.
2. Do not make your contribution more informative than required.

Of course, I can't tell you something I don't know about. So the fact that the "only" thing stolen was my wallet is a mere implicature. Maybe someone else would show up and say:

watashi no mo nusumareta
私のも盗まれた
Mine, too, was stolen.

This wouldn't contradict that "my wallet was stolen." It would, however, cancel the implicature that "only my wallet was stolen": their wallet was stolen, too.

So the ga が lists things exhaustively, except when it doesn't.

A golden rule of this ga が is that it's the opposite of wa は, just like topic-comment and focus-background are (almost) opposites. Observe:

{nusumareta} no wa nani?
盗まれたのは何？
That which {was stolen} is what?
{nusumareta} no wa ore no saifu
盗まれたのは俺の財布
That which {was stolen} is my wallet.

This is even clearer with a noun as topic:

sensei wa dare desu ka?
先生は誰ですか？
The teacher is who?
- sensei wa - topic.
- dare - comment.
- sensei wa watashi desu
  先生は私です
  The teacher is me.
dare ga sensei desu ka?
誰が先生ですか？
Who is the teacher?
- dare ga - focus.
- sensei - background.
- watashi ga sensei desu
  私が先生です
  I'm the teacher.

For reference, some other interrogative pronouns:

itsu ga ii?
いつがいい？
When is good?
- ashita ga ii
  明日がいい
  Tomorrow is good.
doko ga itai?
どこが痛い？
Where is painful?
Where hurts?
- atama ga itai
  頭が痛い
  [My] head is painful.
  [My] head hurts.
dore ga hoshii?
どれがほしい？
Which one is wanted?
Which one do [you] want?
- kore ga hoshii
  これがほしい
  This one is wanted.
  [I] want this one.

It's important to note that "only subjects can be marked with ga."^{(Shibatani 1990, cited in Heycock, 1994)}

This means ga が is only used with subject focus. If you have a question with a non-subject focus, it won't be marked by ga が. Plus, you'll probably have a topic wa は somewhere in the background.

kimi wa nani wo tabeta?
君は何を食べた？
You: what eat did?
You ate what?
What did you eat?
- kimi wa - background, topic, subject.
- nani wo - focus, object.
- tabeta - background, verb.
watashi wa banana wo tabeta
私はバナナを食べた
Me: banana eat did.
I ate the banana.

Above, we have object focus. The focus is simply marked as the object, with wo を.

News

When bringing up news, the ga が particle is used. This happens because of the third type of focus: the sentence focus, which happens you start talking about what "happened" out of nowhere.

With sentence focus, the whole sentence is focus, so there's no background. Since the topic must be part of the background, if there's no background, consequently there's also no topic.

nani ga atta?!
何があった？！
What happened?!
ou ga shinda!
王が死んだ！
The king died!

This function of ga が is called neutral-description.

Although the ga が particle has four functions in total, only two functions can mark the subject: the neutral-description and the exhaustive-listing. So every time you see a ga が marking the subject, it's one of these two.

Also, in the text it's impossible to tell, but there's actually a difference in pronunciation between these two ga が.^{(Kuno, cited in Heycock, 1994, and in Van, 2005)}

ga が with accent: exhaustive-listing.
ga が without accent: neutral-description.

The name of the function comes from the idea that focus (new information) has higher informational value than the background (old information). So if you have a focus and a background in a sentence, it isn't neutral, but if everything is the focus, it's balanced, ~~as all things should be~~.

But wait a minute. The phrase mentions "the king." Obviously, you'd know who "the king" is. So wouldn't that be old information?

No. Because the sentence isn't about the king. The sentence is about "what happened." The whole phrase is new information connected to an unspoken "what happened" background.

ou wa byouki datta
王は病気だった
The king was sick.

Now we have a sentence regarding the king.

As we've seen with the three questions that can all be answered by the same phrase "I ate the banana," the same information can have different foci depending on context.

If we were talking about "what happened," the king isn't topic so it isn't marked as the topic. However, if in a different context someone just came back to the kingdom and asked "I wonder what my good old buddy, the king, is up to now," you could say:

ou wa shinda
王は死んだ
The king died.

It's the same information, but now the topic is the spoken "the king" and not the unspoken "what happened."

It's a common pattern for the information regarding the happening to be introduced with ga が, and subsequent details regarding the individuals involved in that happening to feature wa は instead.

For example, in news articles, the heading or first paragraph often regards "what's new" and will have subjects marked with ga が. The body of the article, however, details the news, who did what, who said what, so it will have stuff marked with wa は.

Another case are stories, fiction. When a story starts, a character can be introduced with ga が, since the sentence informs "what is happening in the story." Afterwards, it informs what the characters did and said, marked with wa は.

For example, here's a story about an oni 鬼:

toaru yama no naka ni, hitori no aka-oni ga sunde-ita.
とある山の中に、一人の赤鬼が住んでいた。
[Once upon a time:] inside a certain mountain, one red oni lived.
aka-oni wa zutto {ningen to naka-yoku nari-tai} to omotte-ita.
赤鬼はずっと人間と仲良くなりたいと思っていた。
The red oni had always felt {[he] wanted to become friends with humans}.
—The first line of a version of the Blue Oni Red Oni story, as found on Wikipedia: 泣いた赤鬼.

When the story above starts, the narrator informs "what happened once upon a time." Since that's what the sentence is about, that's the topic, the "aka-oni" isn't the topic. It's merely a subject. In the next sentence, the narrator details new information about the aka-oni, so he becomes the topic.

Predicate Levels

The neutral description function of the ga が particle can only be used in stage-level predicates.^{(Suzuki 鈴木, 2014)}

This is in reference to the three Carlson classes of predicates:

neko wa doubutsu
猫は動物
Cats are animals.
- Kind-level predicate: the "cat" kind.
kono neko wa kawaii
この猫は可愛い
This cat is cute.
- Individual-level predicate: the "this cat" individual.
neko ga "nyaa" to naita
猫が「ニャー」と鳴いた
The cat made the sound "meow."
The cat meowed "meow."
The cat meowed.
- Stage-level predicate: what happened "now," in this temporal stage.

There are various things to note about this fact.

First off, in the stage-level example, we could have used wa は to mark "the cat" as the topic.

neko wa "nyaa" to naita
猫は「ニャー」と鳴いた
(same meaning.)
- The only difference is that this sentence is about "the cat," while the ga が version is about "what happened."

The important thing is that we couldn't have used the neutral-description ga が in the kind-level and individual-level examples. If you used the ga が particle, it would have the exhaustive-listing function.

Bananas or cats, which ones are animals?
neko ga doubutsu
猫が動物
Cats are animals.
- "Cats" is the focus. This isn't neutral.
This cat or this dog, which one is cute?
kono neko ga kawaii
この猫が可愛い
This cat is cute.
- "This cat" is the focus. This isn't neutral either.

Why does this happen? Basically, because permanent kind and individual properties aren't "news."

The neutral-description function makes the sentence about the hidden "what happened" topic. Therefore, it must be about a new event that happened. A property or trait of individual or kind has permanency.

If you say "cats are animals," they are always animals, all of them, all the time. Even the phrase "this cat is cute" doesn't give the impression anything changed with time. Even if the cat wasn't cute before, that phrase is only about the individual, not about the change.

If the phrase was about change, however, then we could have the neutral-description ga が. For example:

neko ga kawaiku natta!
猫が可愛くなった！
The cat became cute!
- All-new information about "what happened."
- Stage-level predicate.

This restriction becomes most important when talking about individual persons. For example, if we were to say "Tanaka is a foreigner," that's an individual-level predicate concerning what Tanaka is. Therefore, we always use wa は:

Tanaka wa gaikokujin
田中は外国人
Tanaka is a foreigner.

But if we were using a question, we'd use ga が to talk about what he "is" or "isn't."

dare ga gaikokujin?
誰が外国人？
Who is a foreigner?
Tanaka ga gaikokujin
田中が外国人
Tanaka is a foreigner.

Similarly, if we were talking about names:

watashi wa Tanaka
私は田中
I'm Tanaka.
- This is the normal way.
watashi ga Tanaka
私が田中
(same meaning.)
- This would be used if someone was asking "who is Tanaka?" or "which one of you is Tanaka?"

Some resources may simplify the distinction between stage-level and the rest as "temporary attributes" versus "permanent attributes." Although that's true a lot of times, it isn't always true. For example, you can have an individual-level predicate that's not a permanent property.

watashi wa koukousei
私は高校生
I'm a high school student.
- Obviously, I won't be a high school student my entire life, but this predicate is about "me" as an individual. If we were to switch wa は by ga が, it could only be interpreted as exhaustive-listing.

Another tricky part is that verbs in the te-iru form translate to English as "is doing." Although saying "Tanaka is a foreigner" is similar to "Tanaka is sleeping," the former is about the individual, while the latter is about what's happening currently. Right now, he's sleeping. So that's stage-level.

What's happening?
Tanaka ga neteiru
田中が寝ている
Tanaka is sleeping.

This is also the case of this phrase:

What's happening?
ame ga futteiru
雨が降っている
Rain is falling down from the sky.
Rain is raining.
It's raining.

As far as usage goes, this is all you need to know about predicate levels. However, there's something to note that helps explain some theories you can find around about the differences between wa が and ga が.

As you may know, in the Japanese language, nouns feature neither definite and indefinite articles nor plurality.

neko 猫
Cat. (bare singular.)
A cat. (indefinite singular.)
The cat. (definite singular.)
Cats. (bare plural.)
The cats. (definite plural.)

Therefore, whether neko is singular or plural should have nothing to do with the particle marking it.

However, since you can't use the neutral-description ga が with kind-level predicates, and you normally won't use the exhaustive-listing ga が unless you're answering a question either, most of the time you're talking about kinds (all "cats"), you'll use wa は.

neko wa doubutsu
猫は動物
~~Cat is animal~~
~~A cat is an animal.~~
~~The cat is the animal.~~
Cats are animals.
~~The cats are animals.~~

Since a kind-level predicate like above is naturally plural, and you don't use ga が there, the ga が particle tends to mark nouns that aren't translated to plurals: it tends to mark singular nouns only.

monsutaa ga arawareta!
モンスターが現れた！
A monster appeared!

This, of course, is a mere coincidence. You can have a noun marked by neutral-description ga が that ends up being a plural.

What happened?
hito ga shinda!
人が死んだ！
A person died! (singular.)
People died! (plural.)

は + が

The particles wa は and ga が can appear together in a sentence when the topic and the subject are different things.

As we've seen before, when we talk about "what someone did," the subject and topic end up being the same thing.

However, when we talk about one thing, the topic, and the new information, the comment, includes a subject for some grammatical reason, then we have a subject in the comment, and subject and topic end up being different things.

I'll list here the ways this can happen.

Possessed Traits

The comment of a topic can inform about a trait that the topic possesses. For example:

erufu wa mimi ga nagai
エルフは耳が長い
Elves: ears long are.
Elves' ears are long.
The ears of elves are long.

In the phrase above, the topic is "elves," and the new information about elves is that "the ears are long." In other words, "their ears are long."

Naturally, we could also write the phrase above using the no の particle to turn elves into a possessive no の adjective.

erufu no mimi wa nagai
エルフの耳は長い
The ears of elves: long are.
The ears of elves are long.
The elf ears are long.

The only difference between the two is that now the topic is "the ears of elves," not just "the elves." Literally, it depends on whether you're talking about the fantasy race or their ears specifically.

Generally speaking, body parts are often described with wa は and ga が, and they often have idiomatic meanings. For example:

kanojo wa se ga takai
彼女は背が高い
Her: back high is.
She is tall.
kanojo wa atama ga ii
彼女は頭がいい
Her: head good is.
She is smart.
kanojo wa ashi ga hayai
彼女は足が速い
Her: feet fast are.
She's quick on her feet. She's fast.
kanojo wa kuchi ga katai
彼女は口が固い
Her: mouth is hard.
She's tight-lipped. She doesn't spill secrets.

Technically, in phrases such as the above, the subject is linked by a copula to a complement:

mimi ga nagai
耳が長い
The ears are long.
- mimi ga - subject.
- naga~ - complement;
- ~i - copulative suffix of i-adjectives.

The phrase above sounds like it answers the question "what is long?" Well, "the ears are long." In which case we'd have the exhaustive listing function of the ga が particle, with a clear focus-background structure.

mimi ga - focus: new information.
nagai - background: old information.

However, in erufu wa mimi ga nagai, the function of the ga が particle is neutral description. This happens because the entire phrase "mimi ga nagai" has the same informational value.

erufu wa - topic: old information.
mimi ga nagai: comment: new information.

Here, we're reminded of the "news" usage of ga が. For example, if we said:

kanojo wa mimi ga nagaku natta!
彼女は耳が長くなった！
Her: the ears became long!
Her ears became long!
- kanojo wa - old information.
- mimi ga nagaku natta - new information.

And then we removed the topic of that sentence:

mimi ga nagaku natta!
耳が長くなった！
The ears became long!
- All-new information.

We'd have something similar to the "news" usage of the ga が particle.

Opinions

In Japanese, there's a few adjectives and verbs which represent psychological states, including experiences. Such words are used to describe how the topic feels about the subject, or some other psychological relationship.

They never translate literally to English.

watashi wa atama ga itai
私は頭が痛い
Me: head painful is.
"I" regard "head" as "being painful."
My head is painful.
My head hurts.
watashi wa neko ga suki
私は猫が好き
Me: cats liked are.
"I" regard "cats" as "being liked."
I like cats.

Common examples are itai, suki and kirai 嫌い. They're adjectives that mean "is painful," "is liked," and "is disliked," respectively, but, normally, translate to the English verbs "hurts," "likes," and "dislikes."

Another notable adjective is this:

watashi wa kumo ga kowai
私は蜘蛛が怖い
Me: spiders scary are.
"I" regard "spiders" as "being scary."
I'm scared of spiders.
I fear spiders.

Normally, watashi wa kowai 私は怖い translates to "I'm scared." Since watashi is "me" or "I" it says "I'm kowai," so kowai must be "scared," right? Well, no.

Turns out "I'm kowai" is short for "I'm kowai toward something," or "something is kowai to me." Often, you can figure out what this "something" is from context, so the sentence is normally missing that part.

watashi wa anata no kimochi ga wakaru
私はあなたの気持ちが分かる
Me: your feelings understood are.
"I" regard "your feelings" as "being understood."
To me, your feelings are understood.
I understand your feelings.

The verb wakaru means literally "to be understood." So you're saying "something is understood." You don't say "someone understands something" with wakaru. Therefore, the correct is ga wakaru, not wo wakaru.

*kimochi wo wakaru
~~気持ちを分かる~~
(wakaru is intransitive so it doesn't take an object.)

There are other verbs that behave similarly, like mieru 見える, "to be seen," and kikoeru 聞こえる, "to be heard."

watashi wa yuurei ga mieru
私は幽霊が見える
Me: ghosts seen are.
"I" regard "ghosts" as "being seen."
Ghosts are seen to me.
I can see the ghosts.

Conjugations

In Japanese, transitive verbs can become intransitive when conjugated to certain forms. In essence, the verb stops being about a subject doing an action with an object, and starts, instead, describing the subject's relationship with the action.

This happens with the potential form, "is able to do," in which case, the verb no longer describes the action occurs, but the capability of doing the action to something.

Describing "the capability of doing something to something" is similar to describing a "trait" like whether someone's ears are long or whatever. So it can be the comment for a topic.

watashi wa manga wo yomu
私は漫画を読む
Me: manga read.
I read manga.
watashi wa manga ga yomeru
私は漫画が読める
Me: manga is able-to-read.
I can read manga.
- "The power to read manga" is a trait of "me."

On the other hand, the -tai ～たい form," want to do," works just like words describing an opinion of the topic toward the subject as a comment.

watashi wa manga ga yomitai
私は漫画が読みたい
Me: manga is want-to-read.
"I" regard "manga" as "being want-to-read."
I want to read manga.

Grammaticality

Sometimes, whether a sentence is grammatical or not doesn't really matter. For example, take the sentence below:

watashi wa saifu ga nusumareta
私は財布が盗まれた
Me: wallet stolen was.
My wallet was stolen.

Given everything we've seen about topics and subjects, we can safely say that the sentence above is perfectly grammatical.

But just because you're correct that doesn't mean you're right.

If you asked a native speaker about the grammaticality of the sentence above, chances are someone would say it's wrong because it has two subjects. ^{[日本語受け身の質問 - oshiete.goo.ne.jp, accessed 2019-07-03]}

As we've seen before, that's not the case. The wa は particle marks the topic, the ga が particle marks the subject. Why would anyone say it has two subjects?

As we've also seen before, the wa は particle also just happens to mark the subject most of the time. Also most of the time, people hate learning grammar. So you'll eventually come across sentences uttered by natives that look like this:

*watashi wa something wo shitai
私は〇〇をしたい
I want to do something.
(you're supposed to use a ga が there.^{[がしたい～とをしたいの違い - detail.chiebukuro.yahoo.co.jp, accessed 2019-05-17]})
*watashi wa something wo wakaru
私は〇〇を分かる
I understand something.
(you're supposed to use a ga が there, too.^{[「気持ちが分かる」と「気持ちを分かる」 - oshiete.goo.ne.jp, accessed 2019-07-09]})

Their incorrect, but that doesn't mean its not used like that.

On the other hand, watashi wa saifu ga nusumareta makes sense grammatically, but it makes more sense to say it with no の instead:

watashi no saifu ga nusumareta
私の財布が盗まれた
My wallet was stolen.

Subordinate Clauses

The topic of the sentence is what the whole sentence is about, therefore, there's (normally) only one topic per sentence.

And this topic must be in the main clause, not in any subordinate clause.

This means that if you have a subject in a subordinate clause, it'll always be marked with ga が, not with wa は. For example:

neko wa banana wo tabeta
猫はバナナを食べた
The cat: the banana eat did.
The cat ate the banana.
{neko ga tabeta} banana
猫が食べたバナナ
The banana [that] {the cat ate}:

Above, we have one kind of subordinate clause: the relative clause {neko ga tabeta}, which qualifies the noun banana.

The subject is only there to specify the noun—like which banana we're talking about exactly—the subject isn't what the sentence is about, it's only a detail, so it can't be the topic.

watashi wa
{neko ga tabeta} banana wo
mitsuketa
私は猫が食べたバナナを見つけた
Me: the banana [that] {the cat ate} found.
I found the banana [that] {the cat ate}.
- In this sentence, the comment describes what "I," the topic, did.
- What "the cat" did is a mere detail in the greater scheme of things.

Although a relative clause doesn't have a topic inside of it, it sort of has a topic outside of it. After all, relative clauses are "about" the noun they qualify.

neko wa banana wo tabeta
猫はバナナを食べた
The cat: the banana ate.
- The main clause is about "the cat."
{banana wo tabeta} neko
バナナを食べた猫
The cat [that] {ate the banana}.
- The subordinate clause is about "the cat."
{neko ga tabeta} banana
猫が食べたバナナ
The banana [that] {the cat ate}.
- The subordinate clause is about "the banana."
banana wa neko ga tabeta
バナナは猫が食べた
The banana: the cat ate.
- The main clause is about "the banana."

As we've seen before, the topic-comment relationship is extremely vague. The topic can be the agent, or the patient, ending up becoming the subject, or even the object of the main clause.

As we can see in the examples above, the relationship between a relative clause and the noun it qualifies is equally vague. The noun can the subject or the object of its relative clause.

{mimi ga nagai} erufu
耳が長いエルフ
Elves [whose] {ears are long}.

To make matters worse, sometimes the no の particle marks the subject in relative clauses.

{mimi no nagai} erufu
耳の長いエルフ
(same meaning.)

The same rule applies even to pronouns.

ore wa kanojo ga inai
俺は彼女がいない
Me: girlfriend doesn't exist.
I don't have a girlfriend.
- kanojo 彼女
  She. (pronoun.)
  Girlfriend. (noun.)
- iru いる
  To be. To exist. (animate subjects.)
{kanojo ga inai} ore
彼女がいない俺
Me [whose] {girlfriend doesn't exist}.
Me [who] {doesn't have a girlfriend}.
{kanojo no inai} ore
彼女のいない俺
(same meaning.)

Japanese has light nouns and nominalizers that are normally only used together with relative clauses, not as topics. For example, normally koto こと can't be made the topic. (there are some exceptions, but, normally, it can't.)

{watashi ga itta} koto
私が言ったこと
The thing [that] {I said}.

You'll really have to watch out for these light nouns and nominalizers because Japanese grammar relies heavily on them in order to construct more complex sentences.

Every time they show up you'll have a relative clauses qualifying them, and the subject of those relative clauses won't be the topic of the sentence most of the time.

As sentences get more and more complex, it's important to pay attention to what is the focus (or rather, the topic) of the sentence, specially given that both the topic of a sentence and the subject of a relative clause can show up at the very start of the sentence.

watashi wa kekkon shita
私は結婚した
Me: married.
I married.
- This sentence is about "me."
{watashi ga kekkon shita} toki wa haha ga naita
私が結婚したときは母が泣いた
The time [when] {I married}: mother cried.
When {I married}, [my] mother cried.
- This sentence is about "the time when I married."
watashi wa {kekkon shita} toki san-juu-sai datta
私は結婚したとき３０歳だった
Me: the time [when] {married} 30 years old was.
I, [when] {[I] married}, was 30 years old.
I was 30 years old [when] {[I] married}.
- This sentence is about "me," and "the time" is just a detail.

An easy test to figure out whether the subject is part of the subordinate clause is to simply remove the clause altogether and replace it by something else.

~~{watashi ga kekkon shita}~~ toki wa haha ga naita
(...let's remove this part...)
sono toki wa haha ga naita
そのときは母が泣いた
At that time: mother cried.
- This sentence is still about "that time," so this is the topic.
~~watashi wa {kekkon shita}~~ toki san-juu-sai datta
(removing subject...)
sono toki san-juu-sai datta
そのとき３０歳だった
That time [someone] was 30 years old.
- This sentence is about "[someone]." Who? Me? In that case watashi was really the topic!

The noun that you should watch out for the most is the no の. Often, it's called a "particle," but, in this case, specifically, it's a nominalizer used to convert clauses into nouns to fit the grammar syntax of other words.

{banana wo tabeta} no wa dare desu ka?
バナナを食べたのは誰ですか？
{Ate the banana}: who?
Who {ate the banana}?
{neko ga banana wo tabeta} no wa naze desu ka?
猫がバナナを食べたのはなぜですか？
{The cat ate the banana}: why?
Why {the cat ate the banana}?
{neko ga banana wo tabeta} no wa itsu desu ka?
猫がバナナを食べたのはいつですか？
{The cat ate the banana}: when?
When {the cat ate the banana}?

In particular, no の is often combined with other particles that can have rather confusing secondary functions. For example, when combined with ni に, it translates to "even though," and when combined with de で, it translates to "because."

{neko ga banana wo tabeta} no ni
猫がバナナを食べたのに
Even though {the cat ate the banana}, [something happened].
{neko ga banana wo tabeta} no de
猫がバナナを食べたので
Because {the cat ate the banana}, [something happened].

The predicative copulas da だ and desu です become the attributive copula na な in a relative clause. Consequently, the no の sometimes comes after this na な.

kanojo wa kirei desu
彼女は綺麗です
Her: pretty is.
She is pretty.
{kirei na} kanojo
綺麗な彼女
She [who] {is pretty}.
kanojo wa {kirei na} no de moteru
彼女は綺麗なのでモテる
Her: because {is pretty} is popular.
She's popular because {[she] is pretty}.
- moteru モテる
  To be popular. (romantically.)
- In this sentence, the relative clause depends on the topic of the sentence. After all, the subject for the complement kirei is implied to be the topic, "[she]," which is outside the clause.

As if that wasn't enough, the wa は particle can also combine with other particles, like ni に, so you can get stuff like this:

{kanojo ga kirei na} no ni wa riyuu ga atta
彼女が綺麗なのには理由があった
For {her being pretty}: a reason existed.
There was a reason {she is pretty}.
- This sentence is about "for {her being pretty}."

In the sentence above, the wa は marks ni に, which marks no の, which is qualified by a relative clause ending in the na な copula.

Contrast

The contrastive wa は is a second function of the wa は particle, in which it marks the contrastive topic. Literally, a topic is contrastive when it's supposed to contrast with another topic. This normally happen when you have a conjunction, like kedo けど, "but."

neko wa doubutsu da kedo
banana wa doubutsu janai
猫は動物だけど
バナナは動物じゃない
Cats are animals but
bananas aren't animals.

In the example above, the comments of the topics neko and banana contrast with each other.

neko to banana, docchi ga doubutsu desu ka?
猫とバナナ、どっちが動物ですか？
Cats and bananas, which one is animal?
- neko ga doubutsu
  猫が動物
  Cats are animals.
neko to banana wa doubutsu desu ka?
猫とバナナは動物ですか？
Are cats and bananas animals?
- neko wa doubutsu
  猫は動物
  Cats are animals. (but bananas are not.)

The examples above illustrate the difference between the contrastive wa は and the exhaustive ga が.

The exhaustive listing only answers which one is the subject (docchi) for the subject complement (doubutsu).

The contrastive topic implies there's a second topic (banana), with a different comment (not doubutsu).

There's usually no way to tell whether you have just a single topic or one contrastive topic and a hidden topic in a sentence. For example:

Toukyou wa hito ga ooi
東京は人が多い
Tokyo: people many are.
In Tokyo, the people are many.
There's a lot pf people in Tokyo.

If the sentence above has a simple topic, then we're merely explaining new information regarding "Tokyo." If, however, it's a contrastive topic, then we're also implying that "other places don't have so many people."

Marking a word that stands for a time as the topic generally implies a contrast. Observe:

ima kanojo ga inai
今彼女がいない
Now girlfriend doesn't exist.
Right now, [I] don't have a girlfriend.
ima wa kanojo ga inai
今は彼女がいない
Now: girlfriend doesn't exist.
Right now, [I] don't have a girlfriend.
- Implicature: at some other time, not now, I did have a girlfriend.

The implicature produced by such contrast is cancellable. That is, I didn't say "I had a girlfriend before," I merely implied it. So I could say "I never had a girlfriend" to clear things up without sounding like I just said two things that contradict each other.

I used to watch anime. I still do. But I used to, too.

As we've seen before, interrogative pronouns are by function unknowable and can't be the topic. However, it turns out that indefinite pronouns are knowable and can be the topic.

In English, this wouldn't be an issue, but in Japanese, the difference between an interrogative pronoun and an indefinite pronoun is merely a ka か suffix.

dare-ka wa shiranai kedo...
誰かは知らないけど･･･
I don't know who it is, but...
doko-ka wa shiranai kedo...
どこかは知らないけど･･･
I don't know where, but...
itsu-ka wa shiranai kedo...
いつかは知らないけど･･･
I don't know when, but...

The difference between doko どこ, "where," interrogative, and doko-ka どこか, "where," indefinite, is very literally that one is always used to ask "where," while the suffixed variant is used when you don't know "where," but you aren't asking about it either.

doko-ka e ikou
どこかへ行こう
Let's go to "where."
Let's go to "a place I don't know, I can't define, an indefinite place."
Let's go "somewhere."

For example, in the phrase above, we don't know where to go, but we aren't asking for that information either. The fact is that we're going there, wherever it is.

It can be used as the topic because we're referring to some place we can't define, but we know it exists at least. Its existence is old information.

If you say "I don't who it is," you presuppose "it is someone," so you know that much, at least. If you say "I don't know where," you presuppose it must be "somewhere," and so on.

は + は

Whenever you have two wa は in a sentence, you have two topics, and one is going to be contrastive.

kanojo wa atama wa ii
彼女は頭はいい
Her: head: good is.
Her head is good. (but her appearance is bad.)
She is smart. (but she is ugly.)

By saying "the head is good," we have an implicature that "something else is bad." In other words, her head is the only good thing about her. Her looks are bad. Her personality is bad. Her taste in anime is bad. Everything else is bad.

The topic of the sentence is normally the first thing to come in the sentence. Consequently, you normally interpret the second wa は as being the contrast.

neko wa kyou wa banana wo tabeta
猫は今日はバナナを食べた
The cat: today: the banana ate.
The cat ate the banana today.
- neko wa - topic.
- kyou wa - contrastive topic.
- banana wo tabeta - contrastive comment.
- Implicature: the cat, other days, didn't eat the banana.
- Implicature: the cat is supposed to eat the banana every day.
kyou wa neko wa banana wo tabeta
今日は猫はバナナを食べた
Today: the cat: the banana ate.
The cat ate the banana today.
- kyou wa - topic.
- neko wa - contrastive topic.
- banana wo tabeta - contrastive comment.
- Implicature: today, someone else, didn't eat the banana.
- Implicature: every day someone is supposed to eat the banana.

が + が

As I've mentioned before, the ga が particle can only mark subjects, even when it marks the focus, it only marks the subject focus.

You can't have two subjects in a single clause. That means you'll never have two ga が in a single clause. (at least not the ones we've seen so far, anyway.)

But if you wanted to say "the cat and the banana disappeared," that sounds like you have two subjects: "the cat" and "the banana," both doing the "disappeared" thing. So it sounds like you'd want to say this:

*neko ga banana ga kieta
~~猫がバナナが消えた~~
(nope.)

However, grammatically speaking, "the cat and the banana" only counts as one single noun. It's not a noun word, it's a noun phrase, but it still only counts as one.

So when we say "you can only have one subject," we mean "you can only have one noun or one NP (noun phrase) marked as the subject."

To say the NP "the cat and the banana" in Japanese, we need to join the NPs "the cat" and "the banana" with something that means "and." That would be the to と particle.

neko to banana ga kieta
猫とバナナが消えた
The cat and the banana disappeared.

If you have two clauses, you can have two subjects, one per clause, without a problem.

nezumi ga {neko ga tabeta} banana wo mitsuketa
ネズミが猫が食べたバナナを見つけた
The rat found the banana [that] {the cat ate}.

Pure Focus Marker

The biggest problem is that normally when you have a question, you can switch interrogative pronouns in a topic-comment structure for focus-background using the exhaustive-listing function of the ga が particle like this:

TはF?
- {kieta} no wa nani?
  消えたのは何？
  That [which] {disappeared} is what?
FがT?
- nani ga kieta?
  何が消えた？
  What disappeared?

So, given this, you'd imagine a phrase like this:

{ashi ga hayai} no wa dare da?
足が速いのは誰だ？
That [which] {legs are fast} is who?
{Fast} is who?

Could be rewritten as this:

dare ga ashi ga hayai?
誰が足が速い？
Whose "legs are fast"?
Who is "fast"?
- Or, more commonly:
- dare ga ichiban ashi ga hayai?
  誰が一番足が速い？
  Who is number-one legs fast?
  Who is the fastest?

However, as I've been saying time and again, the ga が can only mark the subject. I've quoted Heycock on Shibatani claiming so. So a phrase like that is supposed to have two subjects. You can't have two subjects, so that's wrong. But if that's wrong, why is it not uncommon?

Apparently, the function of ga が isn't just marking the subject. It marks the nominative case, abbreviated NOM. It just happens that the nominative case ends up being the subject most of the time.

So if you have two ga が, you have two NPs marked in the nominative case, rather than two subjects. This NOM-NOM is also called a double-nominative. If it was a munching NOM NOM NOM NOM NOM NOM, it would be multiple-nominative, or whatever.

So a phrase with two ga が isn't grammatically wrong if it has two NPs in the nominative-case, but only one subject.

dare ga ashi ga hayai?
誰が足が速い？
Whose legs are fast?
- dare ga - nominative-case.
- ashi ga - nominative-case, subject.

And, yeah, this is extremely confusing.

Some analyzes even go as far as saying that if you have a double-nominative, one of them is the object of the sentence.^{(Kuno, 1973, cited in Shibatani, 2001)}

boku ga eiga ga suki da
僕が映画が好きだ
I like movies.
- boku ga - subject.
- eiga ga - object.

As we've already seen before, it makes more sense to interpret phrases like above as being an opinion that the topic has, except this time we don't have a topic, we have an NP in the nominative case.

boku ga - nominative-whatever.
eiga ga - nominative-subject.
eiga ga suki da - opinion.

But wait... if we can do this, that means any phrase we could write as wa-ga (topic-subject) can be rewritten as ga-ga (nom-nom).

Before, I said that if you have a subject that is also the topic, you mark it as the topic. We must revise this. If you have something in the nominative case that is also the topic, you mark it as the topic.

watashi wa banana wo tabeta
私はバナナを食べた
I ate a banana.
- watashi wa - topic, nominative-case, subject.
boku wa eiga ga suki da
僕は映画が好きだ
I, movies are liked.
I like movies.
- boku wa - topic, nominative-case.
- eiga ga - nominative-case, subject.

So we basically had double-nominatives all along and we never noticed because we were marking the first nominative with wa は instead.

So it makes sense to think that, whether it's the subject or a non-subject nominative, switching wa は by ga が or vice-versa should work the same way.

In other words, when two ga が are used, the first ga が marks the focus of the sentence (exhaustive-listing), or the entire sentence is focus because there's no topic (neutral-description).

The reason Heycock and Shibatani claim it only marks the subject is because when the focus is the object, an object marker is used instead.

nani wo tabeta?
何を食べた？
Ate what?
- nani wo - focus, object.

So it looks like the focus-marking function of ga が isn't more important than the object-marking function of wo を. Consequently, when ga が marks the focus, the focus must be nominative, and most of the time the nominative case means it's the subject.

dare ga tabeta?
誰が食べた？
Who ate?
- dare ga - focus, nominative-case, subject.

However, when we have a phrase like:

watashi ga ashi ga hayai
私が足が速い
My legs are fast.

The NP watashi isn't an object or anything that can be marked by any other particle. It isn't the subject either. It must be a third thing.

And that third thing is either:

The focus.
- watashi ga - focus.
- ashi ga - subject.
The nominative-case, which is the focus because it's not marked as the topic.
- watashi ga - nominative-case, focus (not topic).
- ashi ga - nominative-case, subject.

In other words, the first ga が is going to be the focus is one way or another.

Personally, I think it's simpler to just say it's the focus, that way you don't have to deal with nominative-cases, and with double-nominatives.

But there's enough research done on double-nominatives today that we can say: yes, it's a thing, and yes, Japanese has them. So I can't just pretend they don't exist.

Omission

In Japanese, the topic and the subject can be omitted from a sentence sometimes for some very complicated reasons that are outside of the scope of this article.

Whenever omission occurs, it's sometimes analyzed as being implicit, or covert, as opposed to explicit, overt.

Other times, it's analyzed as the word being literally replaced by an invisible "null" or "zero," which has special properties and nuance, and is represented by an empty set symbol ∅ or the similar-looking Greek letter phi φ.

Null Particle

The ga が and wa は particles can be dropped out of the sentence sometimes. When this happens, they're said to have been replaced by the "null particle" or "zero-particle," mu-joshi 無助詞.

Although this sounds very complicated, this actually happens way more often than you'd imagine.

Notably, koto aru ことある and koto nai ことない actually feature the null particle between koto こと and the other word.^{[萩原由貴子, 2003. 話し言葉における無助詞―形式的側面を中心として― - lang.nagoya-u.ac.jp, accessed 2019-07-09]}

Similarly:

boku ni wa kankei φ nai
僕には関係φない
To me: relationship nonexistent is.
It has nothing to do with me.

As you may imagine, the null particle φ normally replaces ga が, so that the kankei nai 関係ない above is actually a kankei ga nai 関係がない in disguise.

There are two important things to note about this.

First, some expressions overwhelmingly have the ga が particle dropped. The ones already mentioned, koto aru, koto nai, kankei nai, plus kankei aru 関係ある very commonly feature the null particle.

This means that saying kankei ga aru might sound unnatural at times, since the natural way is omitting it.

Since omission is so common this rises the question regarding whether there's really a particle in there or we're just imagining things. I mean, why not just say kankei-nai is one big word thingy instead of saying there's some sort of invisible, phantom particle in the middle of it?

That's because this same pattern can be seen in a lot of other, random cases.

Remember the two ga が we had before? It's not uncommon for natives to omit the neutral-listing ga が if it comes right after a exhaustive-listing ga が in order to avoid having two ga が one after the other in a sentence. For example:

watashi wa saifu ga nusumareta
私は財布が盗まれた
Me: wallet was stolen.
My wallet was stolen.
{saifu ga nusumareta} no wa dare?
財布が盗まれたのは誰？
That which {wallet was stolen} is who?
Whose {wallet was stolen}?
dare ga saifu ga nuusmareta?
誰が財布が盗まれた？
Whose {wallet was stolen}?
- dare ga - exhaustive-listing ga が.
- saifu ga - neutral-description ga が.
dare ga saifu φ nusumareta?
誰が財布φ盗まれた？
(same meaning.)
- The neutral-description ga が is omitted here.
watashi ga saifu ga nusumareta
私が財布が盗まれた
My wallet was stolen.
watashi ga saifu φ nusumareta
私が財布φ盗まれた
(same meaning.)

Similarly:

dare ga ichiban ashi φ hayai?
誰が一番足φ早い？
Who is the fastest?
nani ga kankei φ aru?
何が関係φある？
What is related?
What has anything to do [with this]?

This doesn't happen only when you have two ga が. Sometimes you'd have just one ga が but it gets dropped anyway.

watashi wa {yatta} koto φ nai
私はやったことφない
Me: the fact {[I] did [it]} nonexistent is.
I never {did [it]}. I've never {done [it]}.

The topic marker wa は can be omitted too. Sometimes it just gets skipped, but there are also cases where it's replaced by a short pause that is represented in manga and novels as a space.

watashi φ yatta koto φ nai
私やったことない
(same meaning.)
watashi, yatta koto φ nai
私　やったことない
(also same meaning.)

Dropping the particles is more common in casual speech compared to formal speech.^{(Tsutsui, 1984, cited in Yatabe, 1999)}

Given this, they aren't much different from contractions and relaxed pronunciation of words.

The neutral-description ga が is kind of neutral and boring, and does nothing, so people wouldn't really care if it gets dropped in a lot of sentences. However, the exhausting-listing ga が is harder to ignore. If it's not there, it makes a noticeable difference in meaning.

Consequently, the exhaustive-listing ga が sounds "unnatural" if dropped.^{(Tsutsui, 1984, cited in Yatabe, 1999)}

Emphasis on the wording "unnatural."

dare φ yatta?
誰φやった？
Who did it?

The phrase above is "unnatural." It sounds weird. Grammatically, however, it's still somehow alright.

Null Pronoun

In Japanese, it's normal to omit the topic when it refers to either "I" or "you." This is sometimes called pronoun dropping. Japanese is a "pro-drop language" because you can drop pronouns out of the sentence.

A notable example is how to say "my name is" in Japanese.

watashi no namae wa Tanaka desu
私の名前は田中です
My name is Tanaka. (too long.)
watashi wa Tanaka desu
私は田中です
I'm Tanaka. (still too long.)
Tanaka desu
田中です
Am Tanaka. (perfect.)

Since the topic of "I'm Tanaka" is "I," we can drop that out because it's implied from context that I'm talking about myself.

Similarly, you don't ask questions to yourself, you ask questions to whom you're talking to, so questions generally have an implicit "you" topic.

anata no namae wa nandesuka?
あなたの名前はなんですか？
Your name: what is? (too long.)
o-namae wa?
お名前は？
Name? (perfect.)
- o- お～
  Polite prefix. Used when referring to things of other people, like their names, for example.

Some of the most well-known Japanese anime words are those that show up in single-word sentences that only exist because of the pronoun dropping. For example:

hayai!
早い！
Fast!
[He] is fast!
kawaii!
可愛い！
Cute!
[This] is cute!
itai!
痛い！
Painful!
[My toe] is painful!
[My toe] hurts!
kowai!
怖い！
Scary!
[This] is scary [to me]!

Just because the topic of a sentence isn't in the sentence, that doesn't mean there isn't one. For example, the two sentences below are about the same thing, so they must be about the same topic.

watashi wa neko ga suki
私は猫が好き
Me: cats are liked.
I like cats.
- watashi wa - topic, old information.
- neko ga suki - comment, new information.
neko ga suki
猫が好き
Cats are liked.
[I] like cats.
- [I] - topic, old information.
- neko ga suki - comment, new information.

In particular, this is problematic when combined with contrastive topics. For example:

watashi wa neko wa suki
私は猫は好き
Me: cats: liked are.
I like cats. (but not dogs.)
- watashi wa - topic.
- neko wa - contrastive topic.
- suki - contrastive comment.
neko wa suki
猫は好き
Cats: liked.
[I] like cats. (but not dogs.)
- [I] - topic.
- neko wa - contrastive topic.
- suki - contrastive comment.

As we've seen before, phrases like neko ga suki express an opinion of the topic watashi wa. Since it has to be the opinion of somebody, there must be a topic somewhere.

If we say neko wa suki instead, the wa は becomes a contrastive wa は, because the notion the phrase is a comment explaining somebody's (the topic's) opinion still remains. You only see one wa は, but it'll behave like you have two wa は.

In the literature, Kuno^{(1973, cited in Shibatani, 2001)} made a similar argument, except it was about double-nominatives instead of two topics. It seems this sort of omission is called elliptical. The ellipsis being the name of those three dots (…) used when part of a phrase is omitted.

As always, it's important to pay attention to what the sentence is about.

okane ga nai kara kaenai
お金がないから買えない
There is no money so can't buy.

In the phrase above, okane is a subject, not a topic. The sentence can't be about okane, because the clause it's subject of, okane ga nai, is merely one part of a larger explanation about something else.

ore wa okane ga nai kara kaenai
俺はお金がないから買えない
Me: there's no money so can't buy.
I don't have money, so [I] can't buy [something].
- The sentence is about "me" now.
- If this was a contrastive topic: other people may have money, so other people can buy it, but I don't, so I can't buy it.
okane ga nai kara kore wa kaenai
お金がないからこれは買えない
There's no money so this: can't buy.
[Someone] doesn't have money so this [they] can't buy.
- Now the sentence is about "this."
- If this was a contrastive topic: they can buy other stuff, but not this.
ore wa okane ga nai kara kore wa kaenai
俺はお金がないからこれは買えない
I don't have money so [I] can't buy this.
- This sentence is about either "I" or "this."
- Something must be contrastive. Either other people have money and can buy it, or other things I can buy.

You'll see that a lot of sentences missing a wa は are actually about something that's found outside the sentence. For example:

uso de aru
嘘である
[That] is a lie.
- [That] - topic, subject.
- uso - complement.
- de aru - copula.

That phrase above could be the same thing as:

sore wa uso de aru
それは嘘である
That is a lie.

Or it could even be something more complicated like:

{kare ga itta} koto wa uso de aru
彼が言ったことは嘘である
The thing [that] {he said} is a lie.

Dissimilar Functions

As mentioned before, the main problem between wa は and ga が is the overlap of topic and focus. That has been explained already. But there are still some functions of wa は and ga が worth noting.

These are dissimilar functions. It's obvious you can't use ga が or wa は instead and vice-versa, so you shouldn't have a problem deciding it and I won't enter in detail about them either. They've been explained in the particles' respective articles already.

The conjunctive ga が translates to "but" or "however." Since wa は can't do this, you'll never have to worry about using wa は instead in this case.

yasui ga oishii
安いが美味しい
Cheap but delicious.

Note that, with a few exceptions, the subject that ga が marks must be a noun. In the phrase above, yasui is an adjective. Therefore, the ga が isn't marking it as the subject, so it must be conjunctive ga が instead.

If we turned the adjective into a noun through the nominalizer no の, then it would be marked as the subject:

yasui no ga oishii
安いのが美味しい
The one [that] {is cheap} is delicious.

The "exception" I'm talking about is the expression -ga ii ～がいい, which can mark clauses. I'm guessing the ga が is the exhaustive-listing ga が marking the focus in this case, but who knows?

kyoufu suru ga ii!
恐怖するがいい！
To fear is good!
Be afraid [of me]! (generally used by villains.)
- ii no wa kyoufu suru
  いいのは恐怖する
  Good is to fear. (?)

Here's a more complex example combining the topic wa は, contrastive wa は, and conjunctive ga が.

watashi wa
neko wa suki da ga
inu wa kirai da
私は
猫は好きだが
犬は嫌いだ
Me: cats are liked but dogs are disliked.
I like cats but [I] dislike dogs.

The genitive-case marker ga が works just like the no の particle, creating no の adjectives, but it's not used in normal Japanese anymore. You'll only see it in pretty much one word plus names of places, and names of people that come from places, in which case it's spelled ヶ.

waga na 我が名
watashi no na 私の名
The name of I.
My name.
Senjougahara 戦場ヶ原
senjou ga hara 戦場が原
senjou no hara 戦場の原
Plains of battlefield.
Battlefield's plains.

The adverbial wa は particle comes after the te-form of verbs and adjectives to create conditions. The te-wa ては can contract into cha ちゃ, and nakute-wa なくては into nakya なきゃ. The ga が particle can't do this, so you won't have to worry about it.

koroshite wa komaru
殺しては困る
koroshicha komaru
殺しちゃ困る
[I'll] be troubled if [you] kill [him].
okane ga nakute wa komaru
お金がなくては困る
okane ga nakucha komaru
お金がなくちゃ困る
okane ga nakya komaru
お金がなきゃ困る
[I'll] be troubled if there is no money.

The topic-marker and contrastive wa は can come between verbs and adjectives and auxiliary verbs and adjectives. The ga が particle can't do this either.

tabete-iru
食べている
[He] is eating.
tabete wa iru
食べてはいる
Eating, [he] is.
warukunai
悪くない
[It] isn't bad.
waruku wa nai
悪くはない
Bad, [it] isn't.

Similarly, the wa は particle can come after a lot of other particles, while the ga が particle can only come after the no の particle and only because the no の nominalizer works like a noun.

kono Joruno Jobaana niwa yume ga aru
このジョルノ・ジョバァーナには夢がある
To this Giorno Giovanna: there is a dream.
This Giorno Giovanna has a dream.
Chikyuu dewa sore ga futsuu
地球ではそれが普通
On Earth: that's normal.

Other examples include towa とは, e wa へは, made wa までは, etc.

The wa は is also seen part of the negative copula and, in rare occasions, the affirmative one:

koko dewanai
ここではない
Here is not.
It isn't here.
koko dewa aru
ここではある
Here is.
It's here.