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い & な adjectives

Japanese has two kinds of adjective. い-adjectives change their own ending; な-adjectives behave like nouns — here's how each one bends.

The short version

An い-adjective ends in and does its own conjugating: たかい → たかくない → たかかった. A な-adjective works like a noun: it needs to touch a following noun (しずかなまち) and だ/です to end a sentence. You never add だ to an い-adjective.

い-adj たかい / たかないたかかったたか — high / not / was / and…
な-adj しず+noun / しずじゃないしずだったしず

い-adjectives — negative & past

Drop the final and add くない for the negative, かった for the past. No です needed to be a full casual sentence.

It's not fun. (楽しい → negative 楽しくない)
It was delicious. (おいしい → past おいしかった)

Joining with 〜くて

To link an い-adjective to what follows ("… and …"), swap い for くて. It chains descriptions or gives a reason.

It's hot and it's rough. (暑い → て-form 暑くて)

な-adjectives — the な that touches a noun

A な-adjective needs before the noun it describes. Otherwise it conjugates exactly like a noun: しずかだ / 静かじゃない / 静かだった.

There was a rough patch. (大変 + な + noun 時期)
いい is a rebel. "Good" conjugates off the older form い, not いい: the negative is よくない and the past is よかった — never いくない / いかった. かっこいい follows the same twist (かっこよかった).

Watch out

🗣 Casual speech drops です/だ. An い-adjective is already a full sentence on its own — あつい!("Hot!"). A な-adjective drops だ too: you'll hear しずか! or きれい! with no ending at all.
Don't mix the two systems. It's たかくない (い-adj), but きじゃない (な-adj, like a noun). If it ends in い but conjugates with な (きれい, きらい), it's secretly a な-adjective.
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