Japanese has two adjective types, and they behave so differently that some textbooks call one of them "adjectival nouns" instead. Telling them apart and conjugating them correctly is one of the first big grammar tasks for a beginner — because you use adjectives constantly, and using the wrong type's rules creates unmistakably wrong sentences.
i-adjectives (い形容詞)
Their dictionary form ends in い, and the い is part of the stem — it changes when the adjective conjugates. They behave almost like verbs: they have past, negative, and past-negative forms built directly onto the word, without needing a copula.
| Adjective | Reading | Meaning |
|---|---|---|
| 高い | takai | tall, expensive, high |
| 新しい | atarashii | new |
| 寒い | samui | cold (weather) |
| 楽しい | tanoshii | fun, enjoyable |
| いい | ii | good (irregular) |
Conjugating i-adjectives
You modify the い ending. Here's the full conjugation table for 高い (takai, "tall"):
| Form | Japanese | Meaning |
|---|---|---|
| Plain present | 高い | is tall |
| Plain past | 高かった | was tall |
| Plain negative | 高くない | isn't tall |
| Plain past negative | 高くなかった | wasn't tall |
| Polite present | 高いです | is tall (polite) |
| Polite past | 高かったです | was tall (polite) |
Notice the polite form is just the plain form with です tacked on — there's no ★高いだ or ★高いでした. The adjective itself carries the tense; です only adds politeness.
na-adjectives (な形容詞)
Behave like nouns. Their dictionary form is the bare word — no い, no な. When they directly modify a following noun (in attributive position), you tack a な between them. When they're predicative (the main "is X" of a sentence), they take the copula です/だ.
| Adjective | Reading | Meaning |
|---|---|---|
| 静か | shizuka | quiet |
| 元気 | genki | energetic, well |
| 有名 | yūmei | famous |
| きれい | kirei | pretty, clean |
| 好き | suki | liked, favorite |
Modifying nouns (attributive)
Drop a な between the adjective and the noun:
| Phrase | Reading | Meaning |
|---|---|---|
| 静かな町 | shizuka na machi | a quiet town |
| 有名な人 | yūmei na hito | a famous person |
| きれいな花 | kirei na hana | a pretty flower |
Predicative — the noun-like behavior
When a na-adjective is the main statement of the sentence, it takes the copula. Tense is on the copula, not on the adjective.
| Form | Japanese | Meaning |
|---|---|---|
| Plain present | 静かだ | is quiet |
| Plain past | 静かだった | was quiet |
| Plain negative | 静かじゃない | isn't quiet |
| Plain past negative | 静かじゃなかった | wasn't quiet |
| Polite present | 静かです | is quiet (polite) |
| Polite past | 静かでした | was quiet (polite) |
See the contrast with i-adjectives: there, the past tense is baked into the adjective (高かった); here, the past tense lives on the copula (静かだった). The adjective itself doesn't change.
Spotting the difference
The great news is most na-adjectives don't end in い — if a word ends in い in dictionary form, it's almost certainly an i-adjective. Almost.
The いい-looking traps
A handful of common na-adjectives end in い but are na-adjectives, not i-adjectives. The most important to know:
| Word | Reading | Type |
|---|---|---|
| きれい | kirei | na-adjective (pretty/clean) |
| 嫌い | kirai | na-adjective (disliked) |
| 有名 | yūmei | na-adjective (famous) |
| 幸せ | shiawase | na-adjective (happy) |
Trying to conjugate きれい as if it were an i-adjective (★きれかった for "was pretty") is one of the most distinctive beginner errors. The correct past is きれいだった.
Adverbs from adjectives
A small bonus: both adjective types have predictable rules for becoming adverbs (modifying verbs instead of nouns).
- i-adjective — drop い, add く. So 早い (early) → 早く (quickly).
- na-adjective — add に. So 静か (quiet) → 静かに (quietly).
| Adjective | Adverb | Example |
|---|---|---|
| 早い (hayai, fast) | 早く (hayaku, quickly) | 早く走る — run quickly |
| 静か (shizuka, quiet) | 静かに (shizuka ni, quietly) | 静かに話す — speak quietly |
You'll use the adverbial forms constantly in real speech. They're the bridge between description and action.