Change い-adjectives: 〜くない · かった · くなかった
Conjugate い-adjectives like self-contained verbs (drop final い for negative 〜くない, past 〜かった, negative past 〜くなかった), plus the essential いい → よい irregular root.
Inflecting い-Adjectives: Drop 〜い, Add Ending
kono hon wa takakunakatta desu yo
This book was not expensive.
い-Adjectives (Inherent Inflection) vs な-Adjectives (Copula Inflection)
Micro-Shiftこの靴は 高くない です。
The adjective itself mutates: 高い → 高くない (never 高いじゃない)These shoes are not expensive.
True い-adjectives conjugate on their stem: drop い and attach 〜くない. Never say '高いじゃない'!
Example Sentences
This book was not expensive.
Today was fun.
Mental Model: The Self-Powering Predicate (Verb-Adjectives)
In Western languages, adjectives are passive words. They rely on helping verbs to carry tense: “It is cheap,” “It was cheap,” “It was not cheap.”
Japanese い-adjectives (形容詞) are fundamentally different: they are self-contained verbal predicates. They have their own internal tense and polarity engines built directly into their final syllable!
- They never conjugate with
*高い ではありませんor*高い でした. - Instead, the adjective itself changes its final い sound to express time and negation.
The Four-Quadrant Conjugation Matrix
Take the adjective 高い (expensive / tall). Drop the final い and attach the quadrant endings:
- Quadrant 1: Present Affirmative (Bare Dictionary Form):
- 高い (“It is expensive.”)
- Quadrant 2: Present Negative (Drop い + くない):
- 高くない (“It is not expensive.”)
- Quadrant 3: Past Affirmative (Drop い + かった):
- 高かった (“It was expensive.”)
- Quadrant 4: Past Negative (Drop い + くなかった):
- 高くなかった (“It was not expensive.”)
Adding です for Politeness
In casual conversation with friends, the plain forms above stand completely on their own: - 昨日は 寒かった。 (“Yesterday was cold.”)
To speak politely to coworkers, teachers, or strangers, simply append です to any of the four forms:
- Polite Present Negative: 高くないです。 (“It is not expensive.”) (Alternative formal): 高くありません。
- Polite Past Affirmative: 高かったです。 (“It was expensive.”)
- Polite Past Negative: 高くなかったです。 (“It was not expensive.”) (Alternative formal): 高くありませんでした。
Critical rule: Notice that です does NOT change into *でした here! The adjective already carries the past tense (高かった); appending *でした would be double-past!
The One Irregular Outlier: いい (Good) Uses Root よい
The single most important exception in Japanese adjective morphology: the everyday word for “good” is いい, but all of its conjugations stem from its classical root よい:
- Present Affirmative: いい (or よい)
- Present Negative: よくない (Never say:
*いくない) - Past Affirmative: よかった (Never say:
*いかった) - Past Negative: よくなかった (Never say:
*いくなかった)
Polite past: よかったです。 (“It was good / That's great!”).
Conjugation
| Base form | Conjugated | Form |
|---|---|---|
| 高い (present) | 高くない (negative) | expensive → not expensive |
| 高い (present) | 高かった (past) | expensive → was expensive |
| 高い (present) | 高くなかった (past neg) | expensive → was not expensive |
| いい (irregular) | よくない (negative) | good → not good (stem turns to よ-) |
In Dialogue
Hana昨日の映画、どうだった？面白かった？
How was yesterday's movie? Was it interesting?
- Ken
Tips and Common Traps
Quick Check
Select the correct form to say: “Yesterday was not cold.” (寒い = cold)