Changes in Pronunciation

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

First, let's take a look at terms for different types of changes in pronunciation in Japanese, and related concepts, then later we'll see how it works. Be warned that some of these terms overlap with each other.

ren'on
連音
Called "sandhi" in English, any change in pronunciation that occurs at morpheme boundary. That is, at the start of a suffix, at the end of a prefix, and so on. Many of the terms in this list are examples of sandhi.
onbin
音便
Changes in pronunciation that affect the conjugation of words.
The altered forms are called onbinkei 音便形.
u-onbin
ウ音便
Changes a kana to u う.
- arigataku ありがたく
  arigatou ありがとう
  Thank you. (expression.)
- tsuyoku 強く
  tsuyou 強う
  Strong.
- samuku 寒く
  samuu 寒う
  Cold.
- yoroshiku よろしく
  yoroshuu よろしゅう
  Very well. Fine. (expression.)
- omohi-ta 思ひた
  omouta 思うた
  Thought. Felt.
i-onbin
イ音便
Changes a kana to i い.
- kaki-ta 書きた
  kaita 書いた
  Wrote.
hatsuonbin
撥音便
Changes a kana to n ん.
This n ん is called hatsuon.
- shini-ta 死にた
  shinda 死んだ
  Died.
sokuonbin
促音便
Changes a kana to the small tsu っ, which represents a "geminate consonant," sokuon 促音.
- tori-ta 取りた
  totta 取った
  Took.
- kachi-ta かちた
  katta かった (勝った)
  Won.
- gaku-kou がくこう
  gakkou がっこう (学校).
  School.
rendaku
連濁
Adds a dakuten 濁点 to the first syllable of a suffix.
- shini-kami しにかみ
  shinigami しにがみ (死神)
  God of death.
handakuonka
半濁音化
Refers to something similar to rendaku that adds a handakuten 半濁点, as in pa-pi-pu-pe-po ぱぴぷぺぽ.
- ichi-hatsu いちはつ
  ippatsu いっぱつ (一発)
  One shot.
renjou
連声
Merges a nasal vowel or a syllable from the rows m~, n~, t~, with a vowel.
- han-ou はんおう
  hannou はんのう (反応).
  Response.
- setsu-in せついん
  settchin せっちん (雪隠)
  Toilet.
renjoudaku
連声濁
Merges a nasal sound or voiced sound and a non-voiced sound together.
- oyogi-ta 泳ぎた
  oyoida 泳いだ
  Swam.
ten'on
転音
Changes a vowel of a syllable.
- 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)
  Coat. Outer garment.
on'in tenka
音韻添加
"Sound addition." Any time a sound is added at the boundary of the morphemes.
- haru-ame はるあめ
  harusame はるさめ (春雨)
  Spring rain.
- ma-naka まなか
  mannaka まんなか (真ん中)
  Very center. Middle.
- ma-shiro ましろ
  masshiro まっしろ (真っ白)
  Pure white.
on'in datsuraku
音韻脱落
"Sound removal." Any time a sound is removed at the boundary of the morphemes.
- hadaka-ashi はだかあし
  hadashi はだし (裸足)
  Barefoot.
- kawa-hara かわはら
  kawara かわら (河原)
  Dry riverbed.
on'in yuugou
音韻融合
"Sound fusion." Any time two sounds are merged together at the boundary of morphemes.
- ki-uri きうり
  kyuuri きゅうり (胡瓜)
  Cucumber.
- kari-udo かりうど
  karyuudo かりゅうど (狩人)
  Hunter.
- nanda kore wa?! なんだこれは？！
  nanda korya?! なんだこりゃ？！
  What is this?!
on'in tenkan
音位転換
Metathesis. Changing the order of the sounds.
- fun-i-ki ふんいき
  fuinki ふいんき (雰囲気)
  Mood. Air (of a a person). Atmosphere (of a place).
- arata-shii あらたしい
  atarashii あたらしい (新しい)
  New.
- Aki-ha-no-hara あきはのはら
  Akiba-hara あきばはら
  Akihabara あきはばら (秋葉原)
  Plains of autumn leaves. (literally.)
sokuon-sounyuu
促音挿入
Geminate consonant insertion. Inserting a sokuon 促音, represented by the small tsu っ, in a word that didn't have it previously. In general, this is just called sokuonbin, but when it's done in a word that's not supposed to have a sokuon, it has an intensifying effect.
- sugoi すごい
  suggoi すっごい
  AMAzing!
- yahari やはり
  yappari やっぱり
  JUST like I thought
youon no chokuonka
拗音の直音化
Turning diphthongs into monophthongs by removing the small kana.
- shujutsu しゅじゅつ (手術)
  shijutsu しじゅつ
  Operation.
chouboin no tanboinka
長母音の短母音化
Turning long vowels into short vowels.
- hontou ほんとう (本当)
  honto ほんと
  Really.
- gakkou がっこう (学校)
  gakko がっこ
  School.
ha-gyou tenkou
ハ行転呼
A historical change in which words spelled with ha-hi-fu-he-ho changed into wa-wi-wu-we-wo, except that wi-wu-we-wo sound the same as i-u-e-o.
- kaha かは
  kawa かわ (川)
  River.
- kohi こひ
  koi こい (恋)
  Love. (romantic.)
- uhe うへ
  ue うえ (上)
  Up.
- kaho かほ
  kao かお (顔)
  Face.

Beyond the terms above, there are also all sorts of unnamed contractions stemming from relaxed pronunciation and so on.

Orthography

Changes in pronunciation occur for various reasons. Mostly, it happens because the way a word is supposed to be pronounced is too difficult to pronounce it that way in practice, so an easier to pronounce pronunciation is normally used instead.

Except that's not exactly true.

To begin with, some changes in pronunciation are exactly that. They're contractions and relaxed pronunciations of words.

sentaku-ki せんたくき (洗濯機)
sentakki せんたっき
Cloth-washing machine.
- We know that sentaku-ki is the way you're supposed to pronounce this by its morphology. Its composed of the word sentaku, which means "to wash clothes," and the suffix ~ki, which means "machine."

In English, this would be like the words "gotcha," "gotta," "gonna," "sorta," "ain't," and so on.

These words are obviously "wrong," in the sense that's not how the word is supposed to be pronounced, that's just how people end up pronouncing it in practice.

A completely separate issue is how the word is supposed to be spelled.

Because if "gotta" is an easier way to pronounce "got to," then people may pronounce it "gotta" even if, in text, it's written "got to."

On the other hand, if you were making a transcript of what someone said, and they said "you gotta concentrate," or something like that, you would write "you've got to concentrate," because that's what the word is supposed to be when correctly pronounced.

On the third hand, if you're writing a story or a comic and you want to emphasize the slurred way a character speaks, you'd write "gotta," not "got to," because the character never says "got to," separately, he always says "gotta," slurred, that's how the character speaks.

Given all this, it's pretty complicated, in English, the relationship between how words are written and how they're spoken. In Japanese, it's even more complicated.

Informal Changes

The English spelling is a mess. It's random. You never know for sure how a word is pronounced until you hear someone say it. Meanwhile, the Japanese spelling is much better, in the sense that the written characters represent the pronounced sounds with far higher precision.

In English, "read" is pronounced differently according to its tense. The katakanization in Japanese tries to match the sound, even though that doesn't happen with the English spelling.
riido リード
I will "read" a book tomorrow.
reddo レッド
I "read" a book yesterday.

Since it can match the sound with such precision, it's much easier to transcribe sounds in Japanese. It's much easier to create onomatopoeia, and other mimetic words. And it's much easier to transcribe word slurs.

For instance, i-adjectives always end in ~i ～い, or at least they're supposed to. There's a common and highly regular slurring that happens with them which commonly gets transcribed as-is. Observe:

~ai becoming ~ee.
- shiranai 知らない
  shiranee 知らねー
  [I] don't know.
- janai じゃない
  janee じゃねー
  Is not.
~oi becoming ~ee.
- sugoi すごい
  sugee すげー
  Incredible.
  Awesome. Amazing.
~ui becoming ~ii.
- darui だるい
  darii だりー
  Sluggish.

Although the proper way to pronounce and spell the words above would be with the ~i ～い ending, it often gets pronounced, and consequently spelled, in the slurred way.

This isn't treated as a misspelling or typo, though. It's an acceptable, although informal spelling.

You wouldn't write an official report using words like these, but if you wrote it on social media, nobody would bother trying to correct you, even though, in English, someone always does that if you write "their" instead of "they're" or vice-versa.

Problematically, although the slurring is easily transcribable with stuff spelled with hiragana, in Japanese, words are mostly spelled with kanji, which really don't tell you how a word is supposed to be pronounced, or how a character actually pronounced a word.

洗濯機 this is read as sentakuki by some people.
洗濯機 this is read as sentakki by some other people.

If you wanted to emphasize the way a character speaks, you would need to use furigana.

Since the kanji represents intelligible words, sometimes a pronounced word is spelled with hiragana if it's unintelligible, or if the author wants to emphasize that the word is pronounced all wrong, in an extremely unusual way, for some reason.

For example, say there's a main character, and he meets a girl, who has a very thick accent, or the girl is a baby, or little child that can't speak right, or has a speech impediment, a lisp, or is speaking with their mouth stuff with something, and so on.

If she says anything, he will hear the sounds, and those sounds will be transcribed, but if he can't make out what word those sounds are supposed to make, the sounds won't be spelled with kanji. If he figures it out later, it will be spelled with kanji later.

With i-adjectives, it's possible to transcribe the slurred pronunciation at the end through okurigana, specially by using a small kana at the end of the word, instead of the normal ~i ～い. For example:

kowai こわい (怖い)
koee こえぇ (怖ぇ)
Scary.
tsuyoi つよい (強い)
tsuee つえぇ (強ぇ).
Strong.
warui わるい (悪い)
warii わりぃ (悪ぃ)
Bad.
Sorry. My bad.

Formal Changes

Some words in Japanese have a change in pronunciation officially built into the word. Such changes are normally mandatory. The word may not be pronounced without the change, or else people won't recognize it, they will think it's a different word.

This is what happens with the onbin, rendaku, handakuonka, renjou, renjoudaku, and ten'on.

For instance, toru 取る means "to take" in Japanese, toru 撮る, "to take a photo," tori 撮り "taking a photo." When prefixed by ji~ 自～, "self," it doesn't become ji-tori, it becomes jidori 自撮り, which means "taking one's own photo."

If you say ji-tori じとり, nobody will know what it means.

If you say jidori じどり, people will know it's about a selfie.

Thus, rendaku is officially built into the word.

It's easy to say that jidori is composed by the morphemes ji 自 and tori 取り respectively, but it's hard to say that it was SUPPOSED to be pronounced ji-tori in first place.

It's a very modern word so we can safely assume that it was never pronounced ji-tori to begin with. It was created already with the change in pronunciation built into it. So it's not like the word changed over time, the morpheme tori changed when it formed the word.

Grammatical Changes

There are countless instances of such thing happening. Some of them are pretty much part of the Japanese grammar.

For instance, the past form of verbs in Japanese is SUPPOSEDLY formed by the ren'youkei plus the ~ta ～た jodoushi, merged through one of the so-called onbin changes.

For example: tori-ta とりた becoming totta とった, "took a photo," is affected by sokuonbin. All godan verbs ending in ~ru ～る follow this pattern.

Consequently, neologisms like guguru ググる, "to google," as in "to search for something on google," are also affected by this change: gugutta ググった, "googled."

It's SUPPOSED to be guguri-ta, except that in practice nobody has ever said guguri-ta, it's always gugutta, always with the change in pronunciation already applied.

This means that in Japanese there are changes in pronunciation that are informal and therefore wrong, and those that are official and therefore right, even though both of them are about basically the same thing: pronouncing a word in an easier way.

Prescribed Lies

Many words in Japanese aren't pronounced the way they're spelled in a dictionary.

The biggest lie you can find are long vowels, which, in katakana words, are represented by a prolonged sound mark, but in native words are sometimes represented by ~i ~い or ~u ～う, coming after certain syllables.

For instance, the word sensei せんせい (先生), "teacher," isn't normally pronounced sense せんせ plus i い, it's pronounced sensee せんせー with a long e え.

sensee センセー
Teacher.
tokei とけい (時計)
tokee トケー
Clock.

Similarly, gakkou がっこう (学校), "school," isn't normally pronounced gakko がっこ plus u う, it's pronounced gakkoo がっこー, with a long ko こ.

nichijou にちじょう (日常)
nichijoo ニチジョー
Daily life.
ojousama おじょうさま (お嬢様)
ojoosama オジョーサマ
Daughter.
Young lady.
Rich girl.

Despite being spelled in the dictionary ending in ~i ～い and ~u ～う, people don't really pronounce that sound separately, they pronounce it as a long vowel.

Furthermore, sometimes people shorten a long vowel that's at the end of a word. This is a different thing entirely. Observe:

hontou ほんとう (本当)
Really. Truly.
- How it's spelled in the dictionary.
hontoo ほんとー
(same meaning.)
- How people pronounce it normally.
honto ほんと
(same meaning.)
- How people pronounce it sometimes.

Intensifying Changes

Some changes in pronunciation add intensity to the word. These are pronounced with a extra geminate consonant somewhere that wasn't supposed to be in the word originally, represented by a small tsu っ. For example:

sugoi すごい
suggoi すっごい
Amazing!

The small tsu っ can't come before h-row syllables, so they turn into p-row syllables through handakuonka.

yahari やはり
yappari やっぱり
As I thought.

This kind of change is different from other changes in pronunciation because it's clearly harder to pronounce the changed word than it's to pronounce the original word. Nevertheless, the meaning of the word stays pretty much the same.

References

日本語の変音現象について - nihongonosensei.net, accessed 2019-11-20.