~i ～い Copula

In Japanese, ~i ～い 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
  テクい
  Skilled. (as in having "technique," tekunikku テクニック)
  (neologism.)
• erai muzukashii
  偉い難しい
  Extremely difficult.
  (adverbial usage.)

Manga: One Punch Man, Wanpanman ワンパンマン (Chapter 10)

• Grammar
  • Copula
    • Contractions
    • Adverbial Usage
  • Neologisms
    • Abbreviations
  • Etymology
• Homonyms
  • Ren'youkei of Godan Verbs
    • Adjective-Like Conjugation of Godan Verbs
  • Particle
• References

Grammar

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.

Copula

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 太郎が若い Tarou is young.	{wakai} hito 若い人 A person [that] {is young}. A {young} person.
na-adjective	kuni ga heiwa da 国が平和だ The country is peaceful.	{heiwa na} kuni 平和な国 A country [that] {is peaceful}. A {peaceful} country.
no-adjective	neko ga doubutsu da 猫が動物だ A cat is an animal.	{doubutsu no} neko 動物の猫 The cat, [which] {is an animal}. The {animal} cat.

An example:

Manga: One Punch Man, Wanpanman ワンパンマン (Chapter 10)

• Context: motion lines everywhere, I can't see a thing!
• hayai!
  速い！
  [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
  国が平和
  The country [is] peaceful.
  • φ - null copula.
• kowa!
  こわ！
  Scary!
  • 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."

Character: The Legendary Yamada Tae 山田たえ
Anime: Zombieland Saga: Revenge (Episode 6)

• Context: a random social media user comments on the legendary Yamada Tae, who is also known by the pseudonym "number 0," zero-gou ０（ゼロ）号.
• zero-gou kami-kawa!!!!!!!
  ０号神かわ！！！！！！！
  Number 0 is super cute!!!!!!!
  • kami
    神
    God. Deity.
    Epic. Super. Top-level. (slang.)
  • kawaii
    可愛い
    Cute. Adorable.
    (kawa is kawaii with the ~ii long vowel clipped off.)

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
  怖いだ
  *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
    「怖い」だ
    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
  怖いです
  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}
  私は蜘蛛が怖い
  {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.

Contractions

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
   • nai ない to nee ねぇ.
   • takai 高い to takee 高ぇ, "high," "expensive."
2. ～ｏい to ～ｅぇ
   • tsuyoi 強い to tsuee 強ぇ, "strong."
   • kakkoii かっこいい to kakkee かっけぇ, "cool."
3. ～ｕい to ～ｉぃ
   • warui 悪い to warii 悪ぃ, "bad."
   • samui 寒い to samii 寒ぃ, "cold."

Manga: Yotsuba to! よつばと！ (Chapter 1, よつばとひっこし)

• koee
  こえー
  (same as...)
  • kowai
    怖い
    Scary.

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
  すごいでかい
  {Incredibly} big.
  (flat adverb.)
  
  • {sugoku} dekai
    すごくでかい
    (same meaning, adverbial form.)

Neologisms

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
  酷い
  Horrible. Cruel. From hidou 非道, "inhuman."
• shindoi
  しんどい
  Tiresome. From shinrou 辛労, "hardship."
• shikakui
  四角い
  Quadrilateral (i.e. rectangular). From shikaku 四角, "quadrilateral (i.e. rectangle)."
• kiiroi
  黄色い
  Yellow (adjective). From ki-iro 黄色, "yellow (noun)."
• gesui
  ゲスい
  Scummy. Sleazy. From gesu 下種, literally "lower sort," as in a "low-life." See also: gesugao ゲス顔.
• emoi
  エモい
  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
  エロい
  Sexy. Hot. From "erotic," erochikku エロチック.
• guroi
  グロい
  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
  ナウい
  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
  ジェラしい
  Jealous. Envious. From "jealously," jerashii ジェラシー.
• tekui
  テクい
  Skilled (in a game, sport). From "technique," tekunikku テクニック.
• puroi
  プロい
  Pro-like. Pro-level. From "pro"," puro プロ.

Abbreviations

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

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

Etymology

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.

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....
  これは････若き日の我が姿････
  This... is my appearance from the days [when] {[I] was young}.
  
  • "was" - the tense of a relative clause may be relative its matrix.
• {ikanaru} genjutsu deshou ka?
  いかなる幻術でしょうか？
  {What sort of} illusion is this?
  {What sort of} witchcraft is this?

Homonyms

Not everything that ends in ~i has the ~i copula. For reference, some examples of homonyms, starting with words that merely end with an ~i ～い syllable:

• hai
  はい
  Yes. (used to agree with whatever the other person said.)
  No. (e.g. "tomatoes aren't fruits, right?" hai = "no.")

Most words are spelled with kanji, while ~i copula is written with hiragana, so it's generally impossible to mistake the ~i copula for an i い syllable, except for words that are sometimes spelled with hiragana for some reason.

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

Ren'youkei of Godan Verbs

The ren'youkei of godan verbs ending in ~u ends in ~i.

• chigai
  違い
  Difference.
  • This is the noun form of chigau 違う, "to differ."
• kai-mono 買い物, "purchased goods," from kau 買う, "to purchase."
• nanno mayoi mo naku なんの迷いもなく, "without any hesitation," from mayou 迷う, "to hesitate."

The ren'youkei of godan verbs ending in ~ru ends in ~ri, but this ending sometimes undergoes i-onbin イ音便, changing it to ~i ～い.

• irassharu
  いらっしゃる
  To be. To come. To go. (sonkeigo 尊敬語 variant of iru いる, kuru 来る, iku 行く.)
• *irasshari-masu
  いらっしゃります
  (undergoes i-onbin.)
• irasshaimasu
  いらっしゃいます
  (same meaning, polite form.)
• irasshaimase
  いらっしゃいませ
  Welcome. (expression used when someone arrives at a store.)
  (meireikei 命令形 (imperative form).)
• irasshai
  いらっしゃい
  (same meaning.)
• tabenasai
  食べなさい
  Eat [it]. (imperative sentence.)
  • ~nasai ～なさい comes from nasaru なさる by the same process irasshai comes from irassharu.

Adjective-Like Conjugation of Godan Verbs

Humorously, the godan verb chigau 違う has been being erroneously conjugated as an i-adjective by youngsters^{(石井, 2011:1)}, which means people saw its ren'youkei chigai and thought it worked so much like an adjective they just turned the word into one.

• chigakute
  違くて
  [That's not it].
  • The te-form of chigau would be chigatte 違って, but chigatte wouldn't be used the same way as chigakute.

Particle

There's an i い particle found at sentence-ending position that's often used to soften the tone of sentences, e.g. sou ka i そうかい, "is that so," nanda i なんだい, "what is it," iku zo i 行くぞい, "let's go," and so on.

Manga: Black Butler, Kuroshitsuji 黒執事 (Chapter 2, その執事、万能)

• Context: Ciel Phantomhive シエル・ファントムハイヴ, a 12 year old boy whose first person pronoun is boku 僕, gets annoyed when called boku, meaning "boy," by a shopkeeper, mostly because his parents died, he decided to become the head of the Phantomhive family, and as such he doesn't want to be treated like a kid.
• irasshai boku, otousan no otsukai ka i?
  いらっしゃいボク　お父さんのお使いかい？
  Welcome, boy, is [it] [your] father's errand? (literally.)
  • Are you here doing an errand for your father?
  • bouten 傍点 - the dots on the furigana besides the word boku ボク, used to indicate emphasis, like bold text in English. In this case, the emphasis is due to Ciel taking offense on the word boku specifically.
• piku'
  ピクッ
  *twitch* (with irritation, in this case.)
  (sound effect.)

References

• Fujiyoshi, N., 1982. On the Phonological System of Japanese Inflections. GENGO KENKYU (Journal of the Linguistic Society of Japan), 1982(81), pp.83-100.
• 坪井美樹, 1997. 形容詞の音便形. 文芸言語研究 言語篇, (32), pp.31-47.
• 石井由希子, 2011. 五段活用動詞 [違う] の形容詞型活用. 千葉大学日本文化論叢, 12.