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Japanese adjectives: い and な
Japanese adjectives conjugate: they carry their own past tense and their own negative, with no help from the verb "to be". "It was cold" is 寒かった — one word, no auxiliary.
There are two families and the distinction takes five minutes to learn. The real difficulty is the three or four traps that textbooks always flag too late, and which you'll find lined up below.
Japanese adjectives conjugate
This is the difference that causes more errors than any other. In English the adjective sits still and the verb "to be" does the moving; in Japanese the adjective itself carries tense and negation: 寒い, 寒くない, 寒かった, 寒くなかった. The verb "to be" never enters those four forms.
The two families, and how to spot them
い-adjectives (寒い, 高い, 楽しい) conjugate on their own. な-adjectives (静か, 有名, 好き) behave more like nouns: they take だ in the plain form and な before a noun. The practical rule is simple — if it ends in い it is almost always the first family — and the exceptions you will actually meet are two: きれい and 嫌い.
な only shows up before a noun
静かだ on its own has no な at all; it appears the moment the adjective leans on a noun: 静かな部屋, "a quiet room". That is why the family is named after a syllable its base form doesn't show, and why the right-hand column of the table is the one worth memorizing.
です adds politeness, not tense
寒いです is the polite version of 寒い. In the past, though, politeness doesn't touch the tense: you say 寒かったです, with the adjective already in the past and です unchanged. This is where the single most common mistake comes from — 寒いでした, which puts the past in the wrong place.
Both families, form by form
The last column is the one the second family is named after: な only appears when the adjective sits in front of a noun. Each form shows its kana reading underneath.
| Adjective | Negative | Past | Past negative | Before a noun |
|---|---|---|---|---|
| 寒いさむいcoldい-adjective | 寒くないさむくない | 寒かったさむかった | 寒くなかったさむくなかった | 寒い日さむいひ |
| 高いたかいtall, expensiveい-adjective | 高くないたかくない | 高かったたかかった | 高くなかったたかくなかった | 高い山たかいやま |
| 楽しいたのしいfunい-adjective | 楽しくないたのしくない | 楽しかったたのしかった | 楽しくなかったたのしくなかった | 楽しい時間たのしいじかん |
| いいgoodい-adjective · trap | よくない | よかった | よくなかった | いい人いいひと |
| 静かだしずかだquietな-adjective | 静かじゃないしずかじゃない | 静かだったしずかだった | 静かじゃなかったしずかじゃなかった | 静かな部屋しずかなへや |
| 有名だゆうめいだfamousな-adjective | 有名じゃないゆうめいじゃない | 有名だったゆうめいだった | 有名じゃなかったゆうめいじゃなかった | 有名な店ゆうめいなみせ |
| きれいだpretty, cleanな-adjective · trap | きれいじゃない | きれいだった | きれいじゃなかった | きれいな花きれいなはな |
| 好きだすきだlikedな-adjective | 好きじゃないすきじゃない | 好きだったすきだった | 好きじゃなかったすきじゃなかった | 好きな音楽すきなおんがく |
The four mistakes everybody makes
These aren't slips: they are what comes out of treating Japanese adjectives like English ones. Seeing them written wrong once helps you stop producing them.
寒いでした寒かったです
The past is already inside the adjective: でした doesn't get added.
いかったよかった
いい conjugates from 良い: every form starts from よ.
きれいいきれいな
It ends in い but it's a な-adjective: before a noun it takes な.
静かくない静かじゃない
な-adjectives don't take くない, they take じゃない (or the formal ではない).
Fix them where they're used
Adjectives go wrong when you produce them, not when you read them. With Yukigo you drill the transformations — negative, past, before a noun — inside real sentences, with spaced repetition and an AI tutor to ask why. Free and ad-free.
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Frequently asked questions
If it ends in い, it is almost always the い family. There are few real exceptions and you will meet two early on: きれい and 嫌い, which end in い but take な. Dictionaries mark な-adjectives explicitly.
Because いい is the colloquial form of 良い (よい), and the conjugation runs off the older form: よくない, よかった, よくなかった. It is the one irregularity worth knowing in the whole category.
Yes, that is the normal polite form. Just watch the past: 寒かったです, never 寒いでした. Politeness is added at the end; the tense stays in the adjective.
With the same logic as the て form of verbs: い-adjectives become 〜くて (安くて美味しい, "cheap and tasty"), な-adjectives become 〜で (静かできれい). You can't just put one after the other.
Grammatically they are close — they take だ and です like nouns — and a few work as both, such as 元気 and 便利. For practical purposes, just remember the な when they stand in front of a noun.