i-Adjective Inflection

i-adjectives are called 形容詞けいようし in Japanese. Unlike English adjectives, 形容詞 behave like stative verbs. There are effectively five inflectional tails, but 形容詞 do not have an imperative form.

Inflectional Tails
Form Inflectional Tail
未然形 ーかろ
連用形 ① ーく
（音便）
② ーかっ
終止形 ーい
連体形
仮定形 ーけれ
命令形

Note: "ー" represents the word stem

Notes

未然形

The mizenkei only has one affix: the non-inflecting suppositional auxiliary verb う.

「美うつくしい」→「美うつくしかろう」 "It's likely to be beautiful."
連用形

The renyoukei is the most productive form. Note that the ーく inflectional tail is very productive.

連用形 Applications
Tail Affixes Notes
ーく＋ ① 用言・
助動詞		 attaches to other inflecting words and auxiliary verbs, e.g. ない or なる
② て・は creates the te-form (て）or the compartive marker は
③ （中止法） used as a conjunctive
ーかっ＋ た, たら, たり only used for the three た affixes
ーウ音便

There are a few special cases where the く inflectional tail undergoes a euphonic change (音便おんびん) due to the sound that follows.

The く sound can instead become a ウ sound instead. You may recongnize the following from the phrases おめでとうございます and うれしゅう存じます.

  • 「めでたい」→「めでとう」
  • 「うれしい」→「うれしゅう」

The rules for ウ音便 are a little less rigid than those for u-verbs. In fact, ウ音便 is not technically required in modern Japanese.

One contrast between adjectives and verbs is the negative form or 否定形ひていけい. The inflecting affix ない is used for both verbs and adjectives, but in different inflectional forms: the negative form of verbs is created in the 未然形, whereas adjectives in the 連用形. One other trick: ない used with verbs is an auxiliary verb, whereas ない used with adjectives is actually an auxiliary adjective!
終止形

Sometimes refered to as the dictionary form becuase all 形容詞 end with an「い」in this form. 終止形 actually means (sentence) final form. It is also an 言い切りいいきり ("statment-final") form: no other words (come after or) attach to this form.

連体形

The 連体形 inflectional tail is an「い」– just like 終止形. When an i-adjective attaches to a noun, it is said to be in prenominal or attributive form (連体形). This form is used to attach to nouns (i.e. "prenominal form").

「まぶしい」→「まぶしい星」"bright star"
仮定形

i-adjectives only take the non-inflecting conditional particle ば in 仮定形.

「悲しい」→「悲しければ」"if your sad"
命令形

i-adjectives do not have an imperative form.

It is possible, however, to create an imperative by attaching the imperative form of する (しろ・せよ) to the continuative form of an i-adjective:

大きい（終止形）→ 「声を大きく(連用形)しろ（命令形）！」