In short

Japanese has two classes of adjective, and every adjective you learn belongs to exactly one of them. い-adjectives (高い takai, “high, expensive”) carry the tense and the negative in their own ending - 高かったです, 高くないです - and attach straight to a noun: 高い山 (takai yama, “a high mountain”). な-adjectives (静か shizuka, “quiet”) do not inflect at all; they behave like nouns, borrow です／でした／じゃないです from lesson 1, and need a な to sit in front of a noun: 静かな部屋 (shizuka na heya, “a quiet room”). Learn each new adjective with its class tag, the way you learn verbs with their group.

The rule

Step 1 - Decide the class. Look at the last kana of the dictionary form.

• If it ends in an い that is not part of a preceding kanji’s reading - 高い takai, 新しい atarashii, 面白い omoshiroi, where 高／新／面白 carry the meaning and the い hangs off in hiragana as okurigana - it is an い-adjective. That い is a live inflectional ending: you will remove it and replace it.
• Everything else you meet as an adjective is a な-adjective: 静か shizuka, 便利 benri, 元気 genki, 親切 shinsetsu. These are really nouns wearing an adjective’s job.

The trap is that a few な-adjectives also end in the -i sound. The ones you will actually meet at A1 are きれい kirei (pretty, clean), 有名 yūmei (famous - written in kanji it shows no い, but its reading is ゆうめい) and 嫌い kirai (disliked). Memorise those three as な-adjectives and you have covered nearly all of the danger. One structural hint helps with the first two: a real い-adjective never ends in the sound えい (-ei), so anything ending in -ei - きれい, ゆうめい, 丁寧 teinei (polite) - is a な-adjective. 嫌い has no such tell; it simply has to be learned.

Step 2 - Put it in front of a noun (attributive use). An い-adjective attaches with nothing between it and the noun. A な-adjective needs な.

い-adj + Noun: 高い山 (takai yama) - a high mountain な-adj + な + Noun: 静かな部屋 (shizuka na heya) - a quiet room

That な is the whole reason the class is called “な-adjective”. No な, and the phrase does not parse.

Step 3 - Put it at the end of a sentence (predicative use). Both classes take です for politeness, but they use it very differently.

• い-adjective: この山は高いです。 (Kono yama wa takai desu.) - This mountain is high. Here です is only a politeness marker - it is not the copula, and it never carries tense. Strip it away and 高い is still a complete plain sentence.
• な-adjective: この部屋は静かです。 (Kono heya wa shizuka desu.) - This room is quiet. The な disappears at the end of a sentence, and です is the real copula from lesson 1, carrying the tense.

Step 4 - Inflect. This is where the two classes fully separate.

い-adjectives change their own ending. Drop the final い to get the stem (高い -> 高), then add:

• negative: stem + くないです -> 高くないです (takakunai desu). The stiffer variant is 高くありません (takaku arimasen).
• past: stem + かったです -> 高かったです (takakatta desu).
• past negative: stem + くなかったです -> 高くなかったです (takakunakatta desu).

Notice that the past lands inside the adjective and です stays unchanged. There is no ×高いでした.

な-adjectives change nothing. They simply take the noun-predicate endings you already know: 静かです／静かじゃないです／静かでした／静かじゃなかったです. If you can conjugate 学生です, you can conjugate every な-adjective in the language for free.

The one irregular: いい. いい ii (“good”) inflects from its older form よい yoi. The non-past is いい, everything else is built on よ:

いいです -> よくないです yokunai desu -> よかったです yokatta desu -> よくなかったです yokunakatta desu. Never ×いくないです, ×いかったです.

This spreads to compounds containing it: かっこいい kakkoii (cool) -> かっこよかった kakkoyokatta.

Step 5 - Link two adjectives. To say “cheap and tasty” you cannot just put two predicates side by side. Each class has its own linking form:

• い-adj: replace い with くて -> 安い -> 安くて. 安くて、おいしいです。 (Yasukute, oishii desu.) - It is cheap and tasty.
• な-adj (and nouns): add で -> 静か -> 静かで. 静かで、きれいです。 (Shizuka de, kirei desu.) - It is quiet and pretty.

Only the last adjective in the chain takes です and carries the tense; everything before it is in くて／で. いい links as よくて yokute, as expected.

好き, 嫌い, 上手, 下手 take が, not を. A small group of な-adjectives describes a feeling or an ability about something. Because they are adjectives and not verbs, the thing involved is marked with が, even where English uses a direct object:

X は Y が 好きです。 - 私は魚が好きです。 (Watashi wa sakana ga suki desu.) - I like fish.

Say ×魚を好きです and you have used the object particle with a word that is not a verb. Same for 嫌い (dislike), 上手 (be good at), 下手 (be bad at), and later 分かる and the potential forms.

あまり and 全然 demand a negative. Two very common degree adverbs are grammatically bound to a negative ending: あまり amari (“not very”) and 全然 zenzen (“not at all”). あまり高くないです。 (Amari takakunai desu.) - It is not very expensive. 全然おいしくなかったです。 (Zenzen oishikunakatta desu.) - It was not tasty at all. Pair both with a negative ending in your own sentences. (You will hear casual 全然大丈夫です, “totally fine”, with a positive - that is a colloquial modern extension of 全然, not the pattern taught here. あまり does appear before a positive, but then it is a different sense - あまり(に) + positive means “excessively”, never “not very”. For the “not very” meaning taught here the negative ending is obligatory.)

Adverbs, in one line (preview). The same stems make adverbs: い-adjectives take く (早い -> 早く hayaku, “early, quickly”), な-adjectives take に (静か -> 静かに shizuka ni, “quietly”). Full treatment in Adverbs of frequency and degree.

Forms & patterns

The master table - every slot, both classes.

The stiffer polite variants (same meaning, more formal - the ないです column above is the everyday one).

The irregular いい／よい.

Core adjectives, tagged by class. Learn the tag with the word.

Examples

Common mistakes

• 静か部屋 (shizuka heya) -> 静かな部屋 (shizuka na heya) - a な-adjective must wear its な to modify a noun. The な vanishes only when the adjective ends the sentence: 部屋は静かです。
• 高いな山 (takai na yama) -> 高い山 (takai yama) - い-adjectives never take な. Their own い already does the attaching, so adding な doubles the job.
• この山は高いでした。 (Kono yama wa takai deshita.) -> この山は高かったです。 (Kono yama wa takakatta desu.) - です carries no tense after an い-adjective. Put the past inside the word: 高い -> 高かった.
• この部屋はきれいくないです。 (Kono heya wa kireikunai desu.) -> この部屋はきれいじゃないです。 (Kono heya wa kirei ja nai desu.) - きれい ends in the kana い but is a な-adjective, so it negates like a noun. Same for 嫌い and 有名.
• 私は日本語を上手です。 (Watashi wa Nihongo o jōzu desu.) -> 私は日本語が上手です。 (Watashi wa Nihongo ga jōzu desu.) - を marks the object of a verb. 上手, 好き, 嫌い and 下手 are adjectives, and what they are about is marked with が.

Tips

• Tag every new adjective the moment you meet it - write it as 静か(な) with the な in brackets, the way dictionaries do. The bracketed な is the entire class label, and it stops the きれい／有名 trap from ever catching you.
• な-adjectives are nouns in disguise. If you already know 学生です / 学生じゃないです / 学生でした / 学生じゃなかったです from Noun predicates with です, you know the whole な-adjective paradigm - just swap the noun. Only the い-class is genuinely new machinery here.
• Chant the い-class as one block: 高い - 高くない - 高かった - 高くなかった. Then swap the stem: 安い, 新しい, 面白い all run through the identical four cells. One drill covers hundreds of words.
• Two small oddities worth recognising. 同じ onaji (“same”) modifies a noun with no な: 同じ本 (onaji hon), not ×同じな本. And 大きい／小さい have alternative attributive forms 大きな／小さな (ōkina / chiisana) used only before a noun - 大きな声 (ōkina koe, “a loud voice”). Both are exceptions to recognise, not patterns to extend.