Japanese adjectives split into two classes — i-adjectives and na-adjectives — and each class conjugates differently, so before you can say not expensive or a beautiful room correctly, you have to know which class your adjective belongs to. The obvious shortcut, "does it end in い?", works most of the time and then betrays you spectacularly. Some of the most common adjectives in the language — きれい, 嫌い, 有名, 幸い, 得意 — end in the sound い and are nonetheless na-adjectives. Conjugate them as if they were i-adjectives and you produce forms no Japanese person ever says (×きれくない, ×嫌かった). This page gives you a test that never lies.
The default: most 〜い adjectives really are i-adjectives
Start with the reassuring part. The great majority of words ending in the kana い are genuine i-adjectives: 高い (expensive/tall), 安い (cheap), 大きい (big), 小さい (small), 新しい (new), 楽しい (fun), 美味しい (tasty). For all of these, the final い is a live grammatical syllable that inflects.
この山はとても高い。
kono yama wa totemo takai
This mountain is very tall.
新しいパソコンが欲しい。
atarashii pasokon ga hoshii
I want a new computer.
So "ends in い → i-adjective" is a decent first guess. The problem is the exceptions, and they are frequent enough that you cannot ignore them.
The trap: na-adjectives that end in the sound い
A small but extremely common set of na-adjectives happen to end in an い sound. You will use several of these every day:
| Word | Reading | Meaning | Class |
|---|---|---|---|
| きれい / 綺麗 | kirei | pretty, clean | na |
| 嫌い | kirai | disliked, hated | na |
| 有名 | yūmei | famous | na |
| 幸い | saiwai | fortunate | na |
| 得意 | tokui | good at (one's strength) | na |
Every one of these takes な before a noun and conjugates with the copula, exactly like 静か or 便利 — never like 高い.
きれいな部屋だね。掃除したばかりみたい。
kirei na heya da ne. sōji shita bakari mitai
What a clean room — looks like it was just tidied up.
嫌いな食べ物は特にありません。
kirai na tabemono wa toku ni arimasen
I don't have any foods I particularly dislike.
Notice きれいな, 嫌いな — the な gives them away. A true i-adjective would never insert な here; you can't say ×高いな山.
The test that never lies: behavior, not spelling
Because the spelling can mislead, identify the class by what the word does, not how it ends. Two quick tests, either of which is decisive.
Test A — the な-insertion test (before a noun)
Put the adjective directly in front of a noun. Does it need な?
- Needs な → na-adjective. きれいな花, 有名な人, 嫌いな科目.
- Attaches bare → i-adjective. 高い山 (no な — ×高いな山 is impossible), 新しい車.
有名な作家の本と、高い辞書を買った。
yūmei na sakka no hon to, takai jisho o katta
I bought a famous author's book and an expensive dictionary.
有名 needs な; 高い attaches bare. One sentence, both behaviors, class settled.
Test B — the negative test (does い drop to く?)
Make it negative. A true i-adjective drops its final い and takes くない. A na-adjective keeps its full shape and uses じゃない / ではない.
- 高い → 高くない (takakunai) — i-adjective, the い became く.
- きれい → きれいじゃない (kirei ja nai) — na-adjective, nothing dropped.
この山は高くないけど、景色はきれいじゃない。
kono yama wa takakunai kedo, keshiki wa kirei ja nai
This mountain isn't tall, but the view isn't pretty.
高い negates to 高くない (い → く); きれい negates to きれいじゃない (untouched). If you ever catch yourself forming ×きれくない, the word must actually be a na-adjective and the test just caught you.
Why the sneaky ones fool you — okurigana vs. reading
Here is the deep reason the trap exists, and once you see it, きれい and 嫌い stop being memorized exceptions and start being predictable.
In a true i-adjective, the final い is written as separate okurigana — a grammatical tail hanging off the kanji, precisely because it inflects. 高い is written 高 + い; when you negate it, the い okurigana is what changes to く (高くない), while the kanji 高 stays put. The い is a detachable, inflecting syllable.
In the sneaky na-adjectives, the い sound is part of the word's fixed reading, not a grammatical tail:
- 綺麗 is read きれい — the い belongs to the reading of 麗 (れい). It is welded in; nothing can peel it off.
- 嫌い is read きらい — the い is part of きらい, the noun-like reading, not an inflecting ending. (Compare the verb 嫌う kirau, "to dislike," a different word.)
- 有名 (ゆうめい), 幸い (さいわい), 得意 (とくい) — the final い-sound is baked into the on-reading or the fixed reading.
Because that い is structural rather than grammatical, it cannot drop. There is no 高→高く-style peeling available, which is exactly why ×きれくない and ×嫌かった are impossible: there is nothing detachable to change. The word has to fall back on the copula (じゃない, だった) instead — the na-adjective pattern.
料理が得意な人がうらやましい。私はあまり得意じゃない。
ryōri ga tokui na hito ga urayamashii. watashi wa amari tokui ja nai
I envy people who are good at cooking. I'm not very good at it.
Watch the contrast inside that sentence: うらやましい is a real i-adjective (an inflecting い), while 得意 is a na-adjective — 得意な人 before the noun, 得意じゃない in the negative. Same い-sound, opposite grammar, and the okurigana logic predicts it.
A note on 嫌 — kanji shared across classes
One more source of confusion: the kanji 嫌 shows up in both 嫌い (きらい, na-adjective, "disliked") and the completely separate feeling-word 嫌 (いや, na-adjective, "unpleasant / no way"). Both are na-adjectives, so the class is stable, but don't let the shared kanji make you think either one inflects like a い-adjective.
そんなの絶対に嫌だ。
sonna no zettai ni iya da
No way, I absolutely don't want that.
Common Mistakes
1. Conjugating きれい like an i-adjective in the negative. きれい is a na-adjective; its negative uses じゃない.
❌ この部屋はきれくない。
kono heya wa kirekunai
Wrong — きれい is a na-adjective; the い can't drop to く.
✅ この部屋はきれいじゃない。
kono heya wa kirei ja nai
This room isn't clean.
2. Making a past-tense ×嫌かった. 嫌い takes the copula's past, だった.
❌ 子供の時、野菜が嫌かった。
kodomo no toki, yasai ga kirakatta
Wrong — 嫌い is a na-adjective; use 嫌いだった.
✅ 子供の時、野菜が嫌いだった。
kodomo no toki, yasai ga kirai datta
When I was a kid, I disliked vegetables.
3. Forgetting な on 有名 before a noun. Its い-ending tempts you to attach it bare like an i-adjective.
❌ 有名歌手に会った。
yūmei kashu ni atta
Wrong — 有名 is a na-adjective and needs な (有名な歌手). (有名 + noun without な reads as a compound, not
✅ 有名な歌手に会った。
yūmei na kashu ni atta
I met a famous singer.
4. Assuming a rare 〜い word must be an i-adjective. When unsure, run the な-test before guessing.
❌ 幸くない結果になった。
saiwakunai kekka ni natta
Wrong — 幸い is a na-adjective; there's no 幸く form.
✅ 幸いなことに、けが人はいなかった。
saiwai na koto ni, keganin wa inakatta
Fortunately, there were no injuries.
Key Takeaways
- Most 〜い words are i-adjectives, but a common set — きれい, 嫌い, 有名, 幸い, 得意 — end in an い-sound yet are na-adjectives.
- Don't identify by spelling; identify by behavior. Put the word before a noun: if it needs な, it's a na-adjective (きれいな). Or negate it: a real i-adjective drops い→く (高くない); a na-adjective doesn't (きれいじゃない).
- The reason the sneaky ones can't inflect: their final い is part of the reading (綺麗＝きれい), not detachable okurigana like a true i-adjective's い (高＋い).
- Watch your negatives and pasts especially — ×きれくない and ×嫌かった are the tell-tale slips.
Now practice Japanese
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Start learning Japanese→Related Topics
- Two Adjective ClassesN5 — Japanese has two structurally different kinds of adjective — い-adjectives that conjugate themselves like verbs, and な-adjectives that are really nouns borrowing the copula — and this single split explains every adjective form you will ever meet.
- na-Adjectives Before a Noun (な)N5 — Why a na-adjective needs な (not だ, not の) to modify a following noun — 静かな部屋 — and the copula-attributive logic that makes the whole class exist.
- i-Adjectives: PresentN5 — The dictionary form of an い-adjective ends in the kana い and works two ways with no helper word — straight before a noun (面白い本) and as a complete predicate ending a sentence (この本は面白い) — because the adjective already contains its own 'to be.'