Japanese adjective conjugation, explained

Japanese has two kinds of adjective and one irregular word. Once you know which kind you are holding, every form follows — including the past tense, which surprises English speakers by living on the adjective rather than on the verb.

I-adjectives (形容詞)

These end in い and do the conjugating themselves. 高い → 高くない (not expensive), 高かった (was expensive), 高くて (expensive and…), 高く (expensively). Because they already carry tense, 高い is a complete sentence — you never say 高いだ.

です is added for politeness only, and it never changes the adjective: 高いです, 高かったです. That is why 高かったです is correct but 高いでした is not.

Na-adjectives (形容動詞)

These behave like nouns. 静か never changes; everything happens to the だ/です that follows it: 静かだ, 静かじゃない, 静かだった, 静かでした. To put one in front of a noun you add な — 静かな部屋, a quiet room. That な is where the name comes from.

The adverb ending is に rather than く: 静かにしてください, please be quiet.

The い trap

Not every adjective ending in い is an i-adjective. きれい (綺麗, pretty/clean), 嫌い (きらい, disliked), 有名 and 元気 are na-adjectives — きれい ends in い only because that is how the word is spelled, and it takes な: きれいな花.

The reliable test is the kanji, not the sound: an i-adjective's final い is okurigana hanging off a kanji stem (高い, 新しい), while きれい's い is part of 麗. When in doubt, each adjective page here states its type at the top.

いい, the one irregular

いい (good) is the only genuinely irregular adjective, and it cheats by borrowing the stem of its older twin よい. The dictionary form is いい, but every other form uses よ-: よくない (not good), よかった (was good — also “thank goodness!”), よくて, よく.

いくない and いかった are not words. This carries over to compounds: かっこいい → かっこよかった.

What each form is for

Verbs work differently — see the verb conjugation guide for those.