Japanese has two major beginner adjective patterns. An い-adjective such as 高い (expensive / tall) changes its own ending. A な-adjective such as 静か (quiet) uses な before a noun and different sentence endings. The final sound is a clue, not proof of the group: きれい and 有名 are な-adjectives.
Before a noun, compare the connection
高い本 means an expensive book. 静かな部屋 means a quiet room. Do not add な to 高い, and do not leave it out between 静か and 部屋. At the end of a polite sentence, use この本は高いです (This book is expensive) and この部屋は静かです (This room is quiet); there is no な before です.
| Meaning | い-adjective: 高い | な-adjective: 静か |
|---|---|---|
| Non-past affirmative | 高いです | 静かです |
| Non-past negative | 高くないです | 静かではありません |
| Past affirmative | 高かったです | 静かでした |
| Past negative | 高くなかったです | 静かではありませんでした |
For an い-adjective, replace the final い: 高い → 高くない or 高かった. 高いでした is incorrect because it mixes the patterns. For な-adjectives, 静かじゃないです is another common spoken negative. Learn one consistent polite pattern first, then recognise alternatives in conversation.
The useful exception: いい
いい means good. Its changed forms use よ: よくない, よかった and よくなかった. 今日の天気はいいです means the weather is good today. 昨日の天気はよくなかったです means it was not good yesterday. Do not build いかった from the surface spelling いい.
Join descriptions without copying English word order
For い-adjectives, use くて to connect: この店は安くて、おいしいです (This restaurant is inexpensive and its food is delicious). For な-adjectives, use で: この部屋は静かで、きれいです (This room is quiet and clean). The final description carries the polite ending.
Meaning depends on the noun. 高い山 is a high mountain, but 高い時計 is an expensive watch. おもしろい本 can be an interesting book; a dictionary entry may also give “funny”, which is not the intended meaning in every context. Store a sample noun with a new adjective.
Describe two places
Imagine a café that is quiet but expensive and another that is lively and inexpensive. For the first, try 静かですが、高いです. For the second, try にぎやかで、安いです. が introduces a contrast here; で connects two descriptions without that contrast.
1. Choose きれい部屋 or きれいな部屋.
きれいな部屋. Although きれい ends in the sound i, it follows the な-adjective pattern.
2. Say “It was cold” using 寒い.
寒かったです. Remove the final い and add かったです; do not say 寒いでした.
3. Make いい negative.
よくないです is a simple polite negative. The changed forms use よ, not い.
4. Connect “quiet” and “convenient”: 静か / 便利.
静かで、便利です. Both are な-adjectives; で connects the first to the next description.