イ音便

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

Explanation

Like all other changes in pronunciation, i-onbin exists because of the following process:

1. You make a word out of two morphemes, like kaki 書き and ta た.
2. Logically, the outcome is the sum of the morphemes, kaki-ta 書きた, and that's how you're supposed to pronounce it.
3. In practice, people pronounce it wrong differently, kaita 書いた, because the way it's "supposed" to be is too difficult to pronounce naturally.
4. The official orthography of the word changes to match its usage in real life, and a linguist comes up with a term for the phenomenon: i-onbin. Not necessarily in this order

連用形

The i-onbin affects the ren'youkei form of godan verbs ending in ku く and gu ぐ, but only under certain circumstances.

In general, the ren'youkei form is derived by changing the vowel of the ending of the verb to ~i. If the case of ~ku ～く, that's ~ki ～き, and for ~gu ～ぐ, that's ~gi ～ぎ.

• sagasu
  探す
  To search.
• sagashi
  探し
  The search.

• kaku
  書く
  To write.
• kaki
  書き
  The writing.

• oyogu
  泳ぐ
  To swim.
• oyogi
  泳ぎ
  The swimming.

There are many auxiliaries that attach to the ren'youkei above.

For example, the jodoushi 助動詞 ~masu ～ます:

• sagashimasu
  探します
  To search. (polite.)
• kakimasu
  書きます
  To write. (polite.)
• oyogimasu
  泳ぎます
  To swim. (polite.)

And the auxiliary adjective ~yasui ～やすい:

• sagashi-yasui
  探しやすい
  Easy to search.
• kaki-yasui
  書きやすい
  Easy to write.
• oyogi-yasui
  泳ぎやすい
  Easy to swim.

The i-onbin only manifests when the jodoushi ta た or te て are suffixed to the ren'youkei.

The ~ta ～た jodoushi creates the past form:

• sagashita
  探した
  Searched.

• *kakita
  書きた
  (wrong.)
• kaita
  書いた
  Wrote.

• *oyogita
  泳ぎた
  (wrong.)
• *oyoita
  泳いた
  (also wrong, since ~gu ～ぐ has dakuten 濁点, ~ta ～た gets dakuten, too.)
• oyoida
  泳いだ
  Swam.
  • The ~da ～だ thing is a separate phenomenon called renjoudaku 連声濁.

While ~te ～て has various functions, like asking people to do things, or connecting to actions together.

• sagashite
  探して
  Search [it for me].
  Search, and.
• kaite
  書いて
  Write [it for me].
  Write, and.
• oyoide
  泳いで
  Swim [for me].
  Swim, and.

Note that i-onbin applies to derived forms of the ta た and te て jodoushi.

For example, the conditional ~tara ～たら jodoushi is derived from ~ta ～た, so i-onbin applies to it, too.

• kaitara
  書いたら
  If wrote.
• oyoidara
  泳いだら
  If swam.

On the other hand, the desiderative ~tai ～たい jodoushi has nothing to do with ~ta ～た, and gets attached to the ren'youkei without i-onbin.

• kakitai
  書きたい
  Want to write.
• oyogitai
  泳ぎたい
  Want to swim.

います

There are a few words that don't fit in the usual rule above, but that normally receive i-onbin when attached to ~masu.

• degozaru
  でござる
  To be.
• degozari
  でござり
  (ren'youkei.)
• degozaimasu
  でございます
  (same meaning, polite.)

• ossharu
  おっしゃる
  To say.
• osshari
  おっしゃり
  (ren'youkei.)
• osshaimasu
  おっしゃいます
  (same meaning, polite.)

According to the dictionary, i-onbin only affects the syllables ~ki ～き, ~gi ～ぎ, ~shi ～し, and ~ri り.

Consequently, the phrase suimasen すいません, "[I'm] sorry," isn't i-onbin, but merely a relaxed pronunciation of sumimasen すみません.

連体形

The i-onbin also seems to have affected the rentaikei 連体形, attributive form, of ku-adjectives and shiku-adjectives, which are kind of archaic, but I'm including them here anyway.

Such attributive form would end in ~ki, like atsuki 熱き, and utsukushiki 美しき, but end up becoming atsui 熱い, "hot," and utsukushii 美しい, "beautiful," instead.

Maybe ku-adjectives and shiku-adjectives are the origin of the modern i-adjective? I don't know for sure, but they seem somehow related.

References

• イ音便 - kotobank.jp, accessed 2019-11-07.