And kanji with manga
Tuesday, April 12, 2022

~i ~い Copula

In Japanese, ~i is a suffix that functions like a copula, which is found in i-adjectives, giving them their name. For example: kawaii かわいい doesn't mean just "cute," it means "to be cute," and we can separate the morphemes into the stem kawai~ meaning "cute," and ~i meaning "to be." This ~i ~い is sometimes suffixed to random stuff to create new adjectives, and, in rare cases, used adverbially:

  • tekui
    Skilled. (as in having "technique," tekunikku テクニック)
  • erai muzukashii
    Extremely difficult.
    (adverbial usage.)
Manga: One Punch Man, Wanpanman ワンパンマン (Chapter 10)


See the article about i-adjectives for conjugation and other grammar about the words that end in ~i. This article is mainly about the suffix itself.


The ~i suffix found in i-adjectives has a function analogous to the da だ copula, na な copula, and no の copula used with na-adjectives and no-adjectives (nouns). Since the latter are copulas, it follows that ~i, too, is a copula. Compare:

shuushikei 終止形 rentaikei 連体形
i-adjective Tarou ga wakai
Tarou is young.
{wakai} hito

A person [that] {is young}.
A {young} person.
na-adjective kuni ga heiwa da
The country is peaceful.
{heiwa na} kuni
A country [that] {is peaceful}.
A {peaceful} country.
no-adjective neko ga doubutsu da
A cat is an animal.
{doubutsu no} neko
The cat, [which] {is an animal}.
The {animal} cat.
Manga: One Punch Man, Wanpanman ワンパンマン (Chapter 10)
  • Context: motion lines everywhere, I can't see a thing!
  • hayai!

    [He] is fast!

The only difference between the copulative suffix ~i ~い and the da だ copula is that da だ can often be omitted, i.e. replaced by a null copula, so it's easier to consider da だ as being separate, while ~i is considered part of the i-adjective word: it's sometimes omitted in exclamations, but that's nonstandard.

  • kuni ga heiwa φ
    The country [is] peaceful.
    • φ - null copula.
  • kowa!
    • This would be kowai 怖い with the ~i suffix clipped. Similarly: sugo, yaba, uma, samu would be clippings of sugoi すごい, "it's incredible," yabai やばい, "it's terrible (oh no!)," umai うまい, "it's delicious," samui 寒い, "it's cold."

In dictionaries, too, the dictionary form of an i-adjective includes the ~i copula, while of a na-adjective doesn't include the da copula.

As one would expect, if ~i functions as a copula, and da functions as a copula, you don't say ~i da ~いだ, because the function of da だ is already performed by ~i ~い.

  • *kowai da
    *It is is scary. (wrong.)
    • This is allowed metalinguistically, e.g. someone asks if a word written somewhere is supposed to be kowai or kawai, you answer:
    • "kowaI" da
      It is "kowai."

In polite speech (teineigo 丁寧語), da だ is generally replaced by desu です. There's no polite copula counterpart for ~i ~い. Instead, desu です is added after ~i. The phrase ~i desu ~いです is allowed even though ~i da ~いだ is not because desu also has the polite function that ~i doesn't have, so it's not redundant.

  • kowai desu
    It is scary. (polite speech.)
    I'm scared.

Note that i-adjectives that refer to mental states, among others, have translations that are difficult to understand in English as a copula due to double subject constructions found in Japanese. Observe:

  • watashi wa {kumo ga kowai}
    {Scary is true about spiders} is true about me.
    {Spiders are scary} is true about me. (here, ~i directly translates to the copula "are.")
    I'm scared of spiders. (here, ~i no longer directly translates, despite the copula "am.")
    • kumo - small subject predicated by kowai.
    • watashi - large subject (and topic) predicated by kumo ga kowai.


There are common contractions that affect the ~i copula, merging it with the preceding syllable to form a long vowel, typically changing the ending to a small ~e ~ぇ or ~i ~ぃ (small kana).

  1. ~aい to ~eぇ
  2. ~oい to ~eぇ
  3. ~uい to ~iぃ
こ・・・こえー・・・・・・ ・・・・・・こえー・・・
Manga: Yotsuba to! よつばと! (Chapter 1, よつばとひっこし)
  • koee
    (same as...)
    • kowai

Adverbial Usage

In rare cases, the ~i copula is used like an adverb, analogous to the ni に adverbial copula, a.k.a. the ren'youkei 連用形 of da だ. Normally, ~ku ~く would be the ren'youkei of ~i, but with extremely few intensifying words, like sugoi すごい, "incredible," you have what's called a flat adverb, where an adjective is used in its adjectival form, rather than its adverbial form, to express its adverbial meaning.

See flat adverbs for details.

Character: Uzaki Hana 宇崎花
Anime: Uzaki-chan wa Asobitai! 宇崎ちゃんは遊びたい! (Episode 1)
  • Context: an anime girl with a lip fang and a shirt spelling in romaji:
  • {sugoi} dekai
    {Incredibly} big.
    (flat adverb.)
    • {sugoku} dekai
      (same meaning, adverbial form.)


The ~i copula is sometimes attached to stuff to create new adjectives. In particular, new i-adjectives, slangs, created out of gairaigo 外来語 loan words are prone end up being spelled in a mixture of katakana カタカナ for the non-Japanese part, and hiragana ひらがな for the native ~i suffix. For example:

  • hidoi
    Horrible. Cruel. From hidou 非道, "inhuman."
  • shindoi
    Tiresome. From shinrou 辛労, "hardship."
  • gesui
    Scummy. Sleazy. From gesu 下種, literally "lower sort," as in a "low-life."
  • emoi
    From "emotional," katakanized emooshonaru エモーショナル.
    Used when referring to emotional, deep, heavy stories, scenes, or when one's feeling emotional, etc.
  • eroi
    Sexy. Hot. From "erotic," erochikku エロチック.
  • guroi
    From "grotesque," gurotesuku グロテスク. More specifically, from the term guro グロ, which refers to body horror (gore, death).
    Used to say a series is gore-ly, or to refer to a body part as being grotesque (e.g. disfigured).
  • naui
    New. Trendy. From "now," nau ナウ.
    (apparently a slang from the 80's that nobody uses nowadays but still survives in media when making references to the 80's.)
  • jerashii
    Jealous. Envious. From "jealously," jerashii ジェラシー.
  • tekui
    Skilled (in a game, sport). From "technique," tekunikku テクニック.
  • puroi
    Pro-like. Pro-level. From "pro"," puro プロ.


Some i-adjectives that mix katakana and hiragana are abbreviations of other i-adjectives:

  • kimoi
    Gross. From kimochi-warui 気持ち悪い.
  • uzai
    Annoying. From uzattai うざったい
  • panai
    Serious. From hanpa nai 半端ない, "not half-hearted."


The ~i ~い copula originates in a ~ki ~き suffix undergoing consonant deletion (or i-onbin イ音便), e.g. you had the word wakaki 若き and the ~k~ consonant was removed (or ki replaced by i), so you ended up with wakai 若い, "young." The ~ki was replaced by ~i (i.e. ~i became more commonly used) after the 13th century, in the early "Kamakura Period," Kamakura-jidai 鎌倉時代.(坪井, 1997:1, citing 桜井, 1966a, b; Fujiyoshi, 1982:88)

This ~ki ~き suffix was the rentaikei 連体形 of adjectives, while a ~shi ~し suffix was the shuushikei 終止形 in what's called "ku conjugation," ku-katsuyou ク活用.

One cool example is yoi よい, "good," which used to be yoki よき and yoshi よし a thousand years ago. You can find still find yoki in dialogues where characters speak archaically, meanwhile yoshi survived as an interjection for "alright" in modern Japanese.

Besides this, there was also a "shiku conjugation," shiku-katsuyou シク活用, which was very similar to ku-conjugation, but while the rentaikei was ~shiki ~しき, the shuushikei was just ~shi ~し, e.g.: the modern utsukushii 美しい, "beautiful," was utsukushiki 美しき and utsukushi 美し. The modern utsukushii has a long vowel (shii しー), while the old one had just a short vowel. Same applies to similar words: kanashii 悲しい, "sad," osoroshii 恐ろしい, "terrifying," and so on.

The terms ku and shiku conjugation come from their ren'youkei 連用形, by the way (e.g. yoku よく, utsukushiku 美しく), which remain the same in modern Japanese.

In modern Japanese, ~i is used for both shuushikei and rentaikei, and both ku conjugation and shiku conjugation were replaced by ~i. Basically the ~ku ending they shared in the ren'youkei started getting replaced by ~i to make their shuushikei and rentaikei. The popularization of ~i started with the rentaikei, and in the "Muromachi Period," Muromachi-jidai 室町時代 (1336–1573), the shuushikei also started ending with ~i, until everything became the uniform conjugation we have today.(坪井, 1997:42, citing 山崎, 1992)

If you see a character using the non-i-ending, ku-and-shiku-conjugations, that could be a hint they're from a period of time before the popularization of ~i, but like yoshi, there are some phrases that have survived containing the old conjugations somehow. The old conjugations can also be used for the poetic factor, because they're so classical-sounding, e.g. in titles and lyrics of songs and so on.

これは・・・・若き日の我が姿・・・・ いかなる幻術でしょうか?
Manga: Ya Boy Kongming!, Paripi Koumei パリピ孔明 (Chapter 1, 孔明、渋谷に降り立つ。)
  • Context: Kongming 孔明, a 3rd century Chinese strategist, sees a mirror for the first time after being reincarnated in the modern world.
  • kore wa.... {wakaki} hi no waga sugata....
    This... is my appearance from the days [when] {[I] was young}.
  • {ikanaru} genjutsu deshou ka?
    {What sort of} illusion is this?
    {What sort of} witchcraft is this?


Not everything that ends in ~i has the ~i copula. A homonym would be the ~i ending found in the ren'youkei of godan verbs ending in ~u.

  • chigai

  • kai-mono物, "purchased goods," from kau 買う, "to purchase."
  • nanno mayoi mo naku なんの迷もなく, "without any hesitation," from mayou 迷う, "to hesitate."

There are also many words that become obviously different when you see that they have kanji—the ~i copula isn't written with kanji.

  • kirei
    Beautiful. Pretty.
    • Is this an i-adjective?
  • kirei
    (same meaning.)
    • Nope. It's a na-adjective.


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