Now that you know the basics of い-adjectives and な-adjectives, it's time to face reality: Japanese adjectives conjugate. The good news is that the patterns are very clean — い-adjectives have one set of rules, and な-adjectives have another (and な-adjective conjugation is identical to noun conjugation, so you get it for free).
い-Adjective: Four Forms
All い-adjectives follow the same rule: drop the final い, then add the appropriate ending.
Using 暑い (hot) as an example:
| Form | Rule | Result | Meaning |
|---|---|---|---|
| Affirmative (present) | 暑い + です | 暑いです | It's hot |
| Negative (present) | 暑 | 暑くないです | It's not hot |
| Affirmative (past) | 暑 | 暑かったです | It was hot |
| Negative (past) | 暑 | 暑くなかったです | It wasn't hot |
Memory shortcut:
- Negative = drop い + くない
- Past = drop い + かった (note the double か sound)
- Negative past = drop い + くなかった
More Practice
高い (expensive / tall):
| Form | Japanese | Meaning |
|---|---|---|
| Affirmative | 高いです | It's expensive |
| Negative | 高くないです | It's not expensive |
| Past | 高かったです | It was expensive |
| Negative past | 高くなかったです | It wasn't expensive |
おいしい (delicious):
| Form | Japanese | Meaning |
|---|---|---|
| Affirmative | おいしいです | It's delicious |
| Negative | おいしくないです | It's not delicious |
| Past | おいしかったです | It was delicious |
| Negative past | おいしくなかったです | It wasn't delicious |
楽しい (fun):
| Form | Japanese | Meaning |
|---|---|---|
| Affirmative | 楽しいです | It's fun |
| Negative | 楽しくないです | It's not fun |
| Past | 楽しかったです | It was fun |
| Negative past | 楽しくなかったです | It wasn't fun |
Key formula: い → くない / かった / くなかった. Memorize these three substitutions and every い-adjective is covered.
な-Adjective: Four Forms
な-adjective conjugation is identical to noun conjugation — if you already learned noun sentences (A は B です), this is free.
Using 有名 (famous) as an example:
| Form | Conjugation | Meaning |
|---|---|---|
| Affirmative (present) | 有名です | It's famous |
| Negative (present) | 有名ではありません | It's not famous |
| Affirmative (past) | 有名でした | It was famous |
| Negative (past) | 有名ではありませんでした | It wasn't famous |
きれい (beautiful):
| Form | Japanese | Meaning |
|---|---|---|
| Affirmative | きれいです | It's beautiful |
| Negative | きれいではありません | It's not beautiful |
| Past | きれいでした | It was beautiful |
| Negative past | きれいではありませんでした | It wasn't beautiful |
Reminder: きれい is a な-adjective, not an い-adjective! It conjugates with です/ではありません/でした/ではありませんでした — NOT by dropping い and adding くない.
Three Word Classes Compared
By now we've learned conjugation for verbs, nouns, い-adjectives, and な-adjectives. Here they are side by side:
| Affirmative | Negative | Past | Negative past | |
|---|---|---|---|---|
| Verb | 行きます | 行きません | 行きました | 行きませんでした |
| Noun | 雨です | 雨ではありません | 雨でした | 雨ではありませんでした |
| な-adjective | 有名です | 有名ではありません | 有名でした | 有名ではありませんでした |
| い-adjective | 暑いです | 暑くないです | 暑かったです | 暑くなかったです |
Key takeaways:
- な-adjectives and nouns conjugate exactly the same way — they follow です
- い-adjectives have their own unique pattern (drop い, add くない/かった/くなかった)
- Verbs have their own ます-form conjugation
The only truly new pattern to memorize is the い-adjective conjugation. な-adjectives just borrow the noun pattern.
Past Time = Past Tense (Mandatory)
This is crucial: if the sentence contains a past time word like "yesterday," "last year," or "ten years ago," the adjective must be in past tense.
| English | Japanese | Note |
|---|---|---|
| Yesterday's exam was hard. | 昨日の試験は難しかったです。 | "Yesterday" → かった |
| This town was lively until 10 years ago. | この街は十年前までにぎやかでした。 | "10 years ago" → でした |
Forgetting to conjugate to past tense creates a contradiction — you said "yesterday" but the adjective is in present tense.
Example Sentences in Action
1. How was the trip? — It was really fun!
旅行はどうでしたか。 とても楽しかったです。
Both use past tense because the trip is over.
2. Was the weather good? — Not really.
天気はよかったですか。 いいえ、あんまりよくなかったです。
(よい has a special conjugation — the next article covers this in detail.)
3. Isn't Japan cold right now? — It's extremely cold!
日本は今寒くないですか。 いいえ、大変寒いですよ。
Summary
- い-adjectives: drop い + くない (negative) / かった (past) / くなかった (negative past)
- な-adjectives: same as nouns — です / ではありません / でした / ではありませんでした
- Past time words in the sentence require past tense conjugation
- Three systems stay separate: verbs follow verb rules, adjectives follow adjective rules, nouns follow noun rules
- な-adjective = noun conjugation, so the only new pattern to memorize is い-adjective conjugation
Self-Check
Q1. Conjugate おいしい (delicious) into negative past tense.
Show answer
おいしくなかったです。
Drop い → おいし + くなかった + です. Meaning: "It wasn't delicious."
Q2. How do you say "This room was quiet" in Japanese? (静か = な-adjective)
Show answer
この部屋は静かでした。
静か is a な-adjective, so past tense uses でした (same as nouns). Never say 静かかったです — that's い-adjective conjugation.
Q3. Which conjugation is WRONG? A. 高い → 高くない B. きれい → きれいくない C. 有名 → 有名ではありません
Show answer
B is wrong.
きれい is a な-adjective, so its negative form should be きれいではありません. きれいくない incorrectly treats it as an い-adjective — the most common mistake learners make.