🌶️ Two kinds of adjectives

Foreign adjectives act like nouns

Adjectives imported from Chinese or English, like 暇 (free, not busy), 元気, 便利, behave exactly like nouns: they take the です set.

暇です I am free · 暇じゃないです I am not free · 暇でした I was free · 暇じゃなかったです I was not free

Native adjectives change themselves

Japanese's own adjectives end in い and carry tense and negative inside the word. After them comes plain です only.

忙しい busy · 忙しくない not busy · 忙しかった was busy · 忙しくなかった was not busy

Same for おいしい, 高い (expensive), 安い (cheap), 暑い (hot), 寒い (cold). The one exception is いい (good): よくない, よかった.