In Japanese, two major groups of verbs are ichidan verbs and godan verbs, and a major doubt of many beginners is: what's the difference between godan and ichidan? How do you tell if a verb is ichidan or godan? What do the words ichidan and godan mean?

The answer for these three questions is conjugation, conjugation, and conjugation. But I suppose I should explain it in more detail.

• Endings
• Conjugation
• Name
• Transitive-Intransitive Verb Pairs

In Japanese, verbs are sometimes divided into group 1, group 2, and group 3, or type 1, type 2, type 3, or class 1, class 2, class 3, ichi-guruupu Iグループ, ni-guruupu IIグループ, san-guruupu IIIグループ, or whatever in the world your teacher, book, resource, material, or blog is calling it now.

Point is: there are three groups of verbs in modern, standard Japanese, which group verbs according to their conjugation, and for some reason are always, consistently enumerated in the following order:

1. godan verbs, also called consonant-stem verbs,
2. ichidan verbs, also called vowel-stem verbs, which always end in ~eru or ~iru.
3. The rest. Which are only two verbs: suru する and kuru 来る, which are also called irregular verbs.

As long as you aren't reading some poem from the 9th century, any verb you find in Japanese will fall into one of the three groups above. In other words, if you know how to conjugate the three groups, you know how to conjugate any verb in Japanese.

See also: ichidan vs. godan verbs.

Among verb types, irregular verbs are verbs whose conjugation is non-standard, and doesn't follow the usual conjugation rules that basically every other verb in the language follows.

The Japanese language is often said to have only two irregular verbs: suru する, "to do," and kuru 来る, "to come," which are also called "group 3 verbs," among the three groups of verbs in Japanese, the other two being godan verbs and ichidan verbs, whose conjugation would be regular.

Besides those, the words aru ある, nai ない, yoi よい, ii いい, and iku 行く also feature irregularities to watch out for. So I'm listing all of them here.

• Grammar
• Conjugation
  
  • 来る
  • する
  • ある
  • 無い
  • 行く
  • いい
  • よい

In Japanese, ichidan verbs are verbs that end in ~eru and ~iru, which undergo ichidan katsuyou 一段活用, "one-column conjugation." This means that, when conjugated, their stem ends at the vowel, ~e or ~i, and it's always that same one vowel, no matter what suffixes are attached to it.

They're also called "group 2 verbs," among the three groups of verbs that exist in Japanese.

• 上一段・下一段
• Conjugation
  
  • Dialects
  • History
  • Auxiliaries

In Japanese, godan verbs are verbs which undergo godan katsuyou 五段活用, "five-column conjugation." This means that, when conjugated, their stem ends at the consonant, and the vowel of the last syllable can change into any of the five vowels: a-i-u-e-o.

They're also called "group 1 verbs," among the three groups of verbs that exist in Japanese.

• Endings
• Conjugation
  
  • Past Form and Te-Form
  • Polite Form
  • ～う Ending
  • ～る Ending

In Japanese, sometimes words are pronounced in a way different from how you'd expect them to be pronounced. This specially happens at the boundary of two morphemes forming a word.

For example, combining the number "one," ichi 一, with the counter for shots, hatsu 発, gets you ippatsu 一発, not ichi-hatsu. Combining the ren'youkei 連用形 form of shinu 死ぬ, shini 死に, with the jodoushi 助動詞 ~ta ～た gets you shinda 死んだ, not shini-ta.

• Types
• Orthography
  
  • Informal Changes
  • Formal Changes
    
    • Grammatical Changes
  • Prescribed Lies
  • Intensifying Changes
• References

In Japanese, long vowels, or "long sounds," chouon 長音, refer to vowels pronounced for twice as long than normal. See also: mora. The opposite are short vowels, or "short sounds," tan'on 短音.

For example, ko こ has a short vowel, while kou こう, koo こー, and koo こぉ have long vowels.

The long vowels are also romanized with a macron, e.g.: Toukyou 東京 becoming Tōkyō, or in other ways, like without macron (Tokyo), or Yuu-gi-ou 遊戯王 becoming Yu-gi-oh.

In Japanese, one mora is the length of time it usually takes to pronounce one kana 仮名, although there are exceptions.

• Grammar
• Rules
  
  • Syllabeme Dialects
• In Poetry
• Japanese Terms
• References

In Japanese, yotsugana 四つ仮名 refers to ji じ, di ぢ, zu ず, and dzu づ. These "four kana 仮名," were originally pronounced distinctly, but in modern Japanese, in some regions of Japan, they're all pronounced the same way, they've become homonyms.

• Usage
• Orthography
  
  • Romanization
• References

In Japanese, handakuonka 半濁音化 is a change in pronunciation similar to rendaku 連濁 in which the syllables ha-hi-fu-he-ho はひふへほ get a handakuten 半濁点, turning into the semi-voiced pa-pi-pu-pe-po ぱぴぷぺぽ.

For example: kinpatsu きんぱつ (金髪), "golden hair, "blond," which combines the morphemes "gold" and "hair," kin きん and hatsu はつ, or ippatsu いっぱつ (一発), "one shot," which combines "one" and "shot," ichi いち and hatsu はつ.

• Examples
• Grammar
• References

In Japanese, tenten 点々 means "dots" or "points." It's a reduplication of ten 点, which means a "dot" or "point."

The word is sometimes used to refer to symbols that look like dots, such as dakuten 濁点 (゛) and the ditto mark (〃), which are also called chonchon ちょんちょん, *striking repeatedly* (phenomime), due to how your hand moves in order to write them.

• miro yo
  見ろよ
  Look!
• koko ni chi no ato mitai ni ten-ten to......
  ここに血のアトみたいに点々と･･････
  In here, [something that] looks like blood marks [is stuck] in drops.
  • ato 跡
    Something left behind by something else, usually as evidence.
    Tracks, traces, marks, scars, etc.
  • tenten to 点々と
    Scattered around as spots, dots, points.

In Japanese, ten'on 転音, also called boin koutai 母音交替, "vowel change," is a change in pronunciation where the vowel of a morpheme changes in order to connect to another morpheme in a word.

Examples include:

• fune-ashi ふねあし
  funaashi ふなあし (舟足)
  Boat speed.
• kaze-kami かぜかみ
  kazakami かざかみ (風上)
  Upwind.
• kami-nagara かみながら
  kamunagara かむながら
  As a god.
  As in the age of gods.
• ame-kasa あめかさ
  amagasa あまがさ (雨傘)
  Umbrella.
• sake-taru さけたる
  sakadaru さかだる (酒樽)
  Alcohol barrel, cask.
• ue-ki うえき
  uwagi うわぎ (上着)
  Outer garment. Coat.
• ki-tachi きたち
  kodachi こだち (木立)
  Tree grove.

Note: in the words above, ka becoming ga, ta becoming da, and ki becoming gi consist of a different change in pronunciation called rendaku 連濁.

In Japanese onbinkei 音便形 is a verb form that has been affected by onbin 音便, by changes in pronunciation. This term is used to distinguish between the original forms and forms that were changed due to euphony.

In general, onbin seems to affect only the ren'youkei 連用形 form of verbs and adjectives. So you have the normal ren'youkei form, and the distorted onbinkei form.

• 五段
  
  • Before ます
• Adjectives
• 四段
• Dialects

In Japanese, onbin 音便, often translated as "euphonic change," refers to four types of changes in pronunciation that affect, among other things, the ren'youkei 連用形 conjugation of verbs. Such changes exist to make the words easier to pronounce. They are:

1. u-onbin
   ウ音便
   Changes a kana to u う.
   Example: omohi-te 思ひて
   Becomes: omoute 思うて.
2. i-onbin
   イ音便
   Changes a kana to i い.
   Example: kaki-te 書きて
   Becomes: kaite 書いて.
3. hatsuonbin
   撥音便
   Changes a kana to n ん. This n ん is called hatsuon.
   Example: shini-te 死にて
   Becomes: shinde 死んで.
4. sokuonbin
   促音便
   Changes a kana to the small tsu っ, which represents a "geminate consonant," sokuon 促音.
   Example: tori-te 取りて
   Becomes: totte 取って.

See onbinkei 音便形 for a more complete list of uses.

According to the dictionary Nihon Kokugo Daijiten 日本国語大辞典, Motoori Norinaga 本居宣長 (1730–1801) was the scholar that introduced the onbin terms to Japanese grammar. When he did so, he also included the rendaku 連濁 in the definition of onbin.^{[音便 - kotobank.jp, accessed 2019-11-10]}

In Japanese, renjoudaku 連声濁 is a change in pronunciation similar to rendaku 連濁, where the first syllable of a suffix gets a "diacritic," dakuten 濁点. Unlike rendaku, renjoudaku merges a nasal or voiced syllable with another syllable, and can affect syllables beyond suffixes.

For example: nado など, "et cetera," comes from nani-to なにと, or nanto なんと. While shinda 死んだ, "died," comes from shini-ta 死にた.

• Definition
• 連用形
• Other Words
• References

In Japanese, renjou 連声 is a change in pronunciation that generally happens when two kanji form a single word, and the last sound of the first kanji is the nasal vowel ~n ～ん, or from the m~ or t~ rows, while the first sound of the second kanji is a vowel, or from the y~ or w~ rows, and they combine to form a syllable from the n~, m~, or t~ rows.

For example: hannou 反応, "response," is formed by han はん and ou おう, but since han はん ends in ~n ～ん, and ou おう starts with a vowel, ou おう becomes nou のう.

• Examples
• References

In Japanese, hatsuonbin 撥音便 is a change in pronunciation of a word where a kana 仮名 is changed to the nasal n ん, also known as hatsuon 撥音. This mainly happens in the conjugation of certain verbs.

For example: shinda 死んだ, "died," is supposed to be shini-ta 死にた, but ni に and ta た are pronounced merged together.

Note: not to be confused with hatsuon 発音, "pronunciation," which is a homonym.

• 連用形
• Nouns
• References

In Japanese, u-onbin ウ音便 is a change in pronunciation where a kana 仮名 becomes u う.

For example: arigatai 有り難い, "unlikely," "thankful," becoming arigatou 有り難う in arigatou-gozaimasu 有難う御座います.

• 連用形
  
  • ～うございます
  • ～うて
• Other Words
• References

In Japanese, i-onbin イ音便 is a change in pronunciation often seen in conjugations where a kana 仮名 of the word gets pronounced as i い instead.

For example: the past form of sagasu 探す is sagashi-ta 探した, but the past form of kaku 書く, isn't kaki-ta 書きた, it's kaita 書いた, with an i い.

• Explanation
• 連用形
  
  • ～います
• 連体形
• References

In Japanese, desushi ですし is the polite copula desu です plus the shi し particle. It works the same way as dashi だし, which has the plain copula da だ instead.

• tetsudatte-kuremasu ka?
  手伝ってくれますか？
  Will [you] help [me]?
• ii desu yo, douse hima desu shi
  いいですよ、どうせ暇ですし
  Fine, since [I] have nothing to do anyway.

The only different, as usual, is when dealing with i-adjectives.

• kawaii shi
  可愛いし
  Since [it] is cute.
• kawaii desu shi
  可愛いですし
  (polite variant of the above.)
• kawaii da shi
  可愛いだし
  (this is wrong, because da だ can't come immediately after an i-adjective, even though desu です can.)

In Japanese, dashi だし is the da だ copula plus the shi し particle. The shi し particle is used to list facts, reasons, and so on, and it comes after the predicative form of words.

For some words, the predicative form ends in the da だ copula, that's when it forms dashi だし, but for other words, like i-adjectives, it does not, and you end up with just shi し.

Beware of homonyms: dashi 出し, "putting out," the noun form of dasu 出す, "to put out," can also mean a Japanese kind of soup stock.

• Context: regarding Komi-san 古見さん.
• bijin dashi,
  atama mo ii shi,
  ninkimono dashi,
  美人だし、頭もいいし、人気者だし、
  [She] is a beautiful-person, [she] is also smart, [she] is a popular-person,
  • atama ga ii 頭がいい
    Literally "head is good."
    To be smart.
  • Here, shi し comes after words in their predicative forms.
  • For adjectives like bijin and ninkimono, those forms are bijin da and ninkimono da, with the predicative copula da だ.
  • For an i-adjective like ii いい, "good," the da だ copula isn't necessary, so shi し comes right after it.

In Japanese, the shi し particle is a conjunctive particle used to express the reason, or reasons, for something. More generally, it's used to emphasize facts regarding something.

For example: in douse hima da shi どうせ暇だし, the shi particle expresses that douse hima da, "I'm free," in the sense of "I don't have anything better to do anyway," is the reason for doing something.

Not to be confused with the homonyms: shi し, the ren'youkei 連用形 form of suru する, shi 死, "death," or shi 四, the number "four."

• Grammar
• References

An "auxiliary" in Japanese might be any of the following things:

1. jodoushi 助動詞
   Morphemes suffixed to verbs and adjectives in order to inflect them, like ~tai ～たい.
2. Auxiliary adjectives.
   Including suffixes of compound adjectives, and adjectives like ii いい,"good."
3. Auxiliary verbs.
   Including suffixes of compound verbs, and verbs like suru する, "to do."
4. hojo-yougen 補助用言
   An umbrella term for auxiliary words that go after the te-form of verbs: the hojo-doushi 補助動詞 and hojo-keiyoushi 補助形容詞.
5. keishiki-meishi 形式名詞
   Formal nouns, like koto こと.
6. rengo 連語
   "Collocations" are sometimes referred to as auxiliaries, like kamoshirenai かもしれない, "might be," which is composed by the particles ka か, mo も, and the verb shiru 知る in its negative potential form.

In Japanese, auxiliary adjectives are auxiliaries that work like adjectives. This single English term, "auxiliary adjective," has been used to refer to various things in Japanese, like:

1. hojo-keiyoushi 補助形容詞
   "Support adjectives."
   Adjectives that come after the te-form of a verb, or after the adverbial form of an adjective.
   Example: in tabete-hoshii 食べてほしい, "[I] want [you] to eat [it]," the auxiliary adjective hoshii ほしい.
2. fukugou-keiyoushi 複合形容詞
   "Compound adjectives."
   The ones where an adjective is attached to the noun form of a verb.
   Example: in tabe-yasui 食べやすい, "easy to eat," the auxiliary adjective yasui やすい.
3. jodoushi 助動詞
   "Helper verbs."
   The ones which inflect like adjectives.
   Example: in tabetai 食べたい, "want to eat," the ~tai ～たい suffix.
4. The adjective ii いい.
   Just... the adjective ii いい. And its conjugations.
   It doesn't fall in any of the categories above, but it's practically an auxiliary.
   
   

In Japanese, the term hojo-keiyoushi is sometimes used to refer to words that aren't technically hojo-keiyoushi. As far as I know, only hoshii, and perhaps nai ない, are hojo-keiyoushi. Everything else is something else.

In Japanese, auxiliary verbs are auxiliaries which are verbs. This single English term, "auxiliary verb," has been used to refer to various, completely different things in Japanese, like:

1. hojo-doushi 補助動詞
   "Support verbs."
   Verbs that come after the te-form of another verb, or the adverbial form of an adjective.
   Example: in tabete-iru 食べている, "to be eating," the verb auxiliary verb ~iru ～いる.
2. fukugou-doushi 複合動詞
   "Compound verbs."
   The ones composed of two verbs, where the suffix is the auxiliary verb.
   Example: in tabe-yagatta 食べやがった, "dared to eat [it]," the auxiliary verb ~yagaru ～やがる.
3. jodoushi 助動詞
   "Helper verbs."
   Verbs, and other morphemes, that come after one of the basic forms of another word.
   Example: in tabesaserareru 食べさせられる, "to be forced to eat [it]," the auxiliaries saseru and rareru.
4. Light verbs.
   Like suru する, its irregular potential form, dekiru できる, aru ある, and naru なる.

In Japanese, sometimes the term hojo-doushi is used, perhaps mistakenly, to refer to the suffixes in fukugou-doushi, and even to some jodoushi, which, as far as I'm aware, aren't technically hojo-doushi, they just kind of work like auxiliary verbs.

In Japanese, jodoushi 助動詞 are the morphemes suffixed to the six basic forms of verbs and adjectives in order to conjugate them into more complex forms.

For example, in nomaserarenakatta 飲ませられなかった, "[I] didn't let [you] make [me] drink [it]," there are four jodoushi: the past ta, the negative nai, the passive rareru, and the causative saseru, thus, the verb nomu 飲む, "to drink," is in its past, negative, passive, causative form.

In English, jodoushi is often confusingly translated as "auxiliary verb." There are various types of auxiliary verbs in Japanese, and jodoushi are the ones that least look like verbs. I mean, nai is an adjective, and even the ~ta ～た of past forms is a jodoushi.

In dictionaries, it's abbreviated to jodou 助動.

• List
• Grammar
  
  • Pronunciation
  • Agglutination
  • Copulas
• Other Auxiliaries