English adjectives don't change. "Expensive" is expensive in every tense, positive or negative, and the verb carries the grammar. Japanese splits the job differently: い-adjectives conjugate like verbs, and な-adjectives behave like nouns. Getting the two classes mixed up is the single most common beginner error, and it's completely avoidable.
Telling them apart
い-adjectives end in い in dictionary form and that い is part of the word: 高い (expensive), 新しい (new), 楽しい (fun), いい (good).
な-adjectives need な to attach to a noun: 静かな部屋 (a quiet room), 有名な人 (a famous person), 便利な (convenient), 元気な (healthy).
The trap: a handful of な-adjectives also end in い. The ones you'll actually meet are きれい (pretty/clean), 嫌い (disliked), 有名 (famous), 丁寧 (polite/careful), and 幸い (fortunate). They take な and conjugate like nouns: きれいな人, not きれい人 — and never きれくない.
For most of them the writing gives it away: the final い is inside the kanji reading rather than an inflecting tail — 有名 is ゆうめい as a unit, with nothing to conjugate, unlike 高い. 嫌い is the genuine exception (it comes from the verb 嫌う), so just memorize that one. There are only a few; they're a spelling coincidence, not a category.
Conjugating い-adjectives
Drop the い and add the ending. Note that です does not conjugate here — the adjective does.
| Form | Example (高い) | Meaning |
|---|---|---|
| Present | 高いです | is expensive |
| Negative | 高くないです | isn't expensive |
| Past | 高かったです | was expensive |
| Past negative | 高くなかったです | wasn't expensive |
| Linking (て) | 高くて | expensive and… |
| Adverbial | 高く | expensively |
Common error: 高いでした. The past goes on the adjective (高かった), not on です.
いい is irregular. It conjugates from its older form よい: いい → よくない → よかった → よくなかった. Never いくない.
Conjugating な-adjectives
These behave like nouns — です carries the grammar, exactly as it does after a noun.
| Form | Example (静か) | Meaning |
|---|---|---|
| Present | 静かです | is quiet |
| Negative | 静かじゃないです | isn't quiet |
| Past | 静かでした | was quiet |
| Past negative | 静かじゃなかったです | wasn't quiet |
| Linking (て) | 静かで | quiet and… |
| Adverbial | 静かに | quietly |
| Before a noun | 静かな部屋 | a quiet room |
Compare directly: 静かでした (な-adj, です carries the past) vs 高かったです (い-adj, the adjective carries it). That contrast is the whole lesson.
Linking two adjectives
- い-adjective: drop い, add くて → 安くておいしい (cheap and tasty)
- な-adjective: add で → 静かできれい (quiet and pretty)
Same job as the te-form of verbs, and it's the natural way to stack descriptions instead of repeating です.
Making adverbs
- い-adjective: い → く → 早い → 早く (quickly)
- な-adjective: + に → 静か → 静かに (quietly)
The fastest way to internalize this
Don't drill the tables. Two conjugation grids are easy to read and hard to retrieve at conversation speed — what actually transfers is having heard the correct form dozens of times in context.
So the practical route is: understand the rule once (you just did), then get volume of exposure to real sentences using both classes. Every adjective you learn should arrive inside a sentence, in its conjugated form, with audio — that's what makes 高かったです feel wrong-if-changed rather than requiring a lookup.
That's how Cotobara presents vocabulary: each word with a level-appropriate example sentence and natural-sounding audio, with furigana you can switch off, so the conjugation patterns get learned the way native patterns are learned — by repeated correct exposure. Adjectives are heavily represented in the N5 and N4 sets; see essential N5 vocabulary and the full N5 grammar list.
Start Cotobara for free — learn adjectives in sentences, not in tables.