i-Adjectives: Negative (〜くない)

Negating an い-adjective is one rule, cleanly applied: drop the final い and add くない. 高い becomes 高くない, 安い becomes 安くない, 面白い becomes 面白くない. There are no exceptions in the pattern itself (only the word いい hides a stem swap, covered below). The single thing that goes wrong for English speakers is reaching for じゃない — the negative they learned for nouns — and gluing it onto an adjective. That produces ×高いじゃない, which no native speaker says. This page shows you the correct pattern, why the く is there, and why the negative you build here quietly becomes an い-adjective in its own right.

The rule: drop い, add くない

Take the dictionary form, remove the final い, and attach くない. Notice you only touch the okurigana — the kanji stem never changes.

Why く? The adverbial stem is doing the work

The く is not random. Dropping い and adding く gives you the adverbial (continuative) form of the adjective: 高く means "highly / in a high way," 安く means "cheaply." That く-form is a connector — it is how an adjective hooks onto whatever comes next. What comes next in the negative is the word ない ("not existing / not being"). So 高くない is literally "high-ly + not" → "not high."

This is worth internalizing because the same く-stem shows up everywhere: 高くなる (to become expensive), 安く買う (to buy cheaply), 面白くて (interesting, and…). The negative isn't a special one-off ending; it is the adverbial stem plus the word ない. Learn く as "the form an adjective takes before it attaches to something," and the negative stops being a thing to memorize.

Polite negative: two correct options

There are two standard polite negatives, and both are completely correct:

1. 〜くないです — take the plain 〜くない and add です for politeness. Common, everyday polite speech.
2. 〜くありません — a slightly more formal alternative; ありません is the polite negative of ある, matching the ない inside 〜くない.

Register note: 〜くありません (formal) reads as a touch more careful or written; 〜くないです (informal-polite) is what you will hear most in ordinary conversation. Both are safe with anyone. The plain 〜くない (informal) is for friends and family.

The hidden payoff: ない is itself an い-adjective

Here is the fact that unifies the whole negative system: the ない on the end of 高くない is an い-adjective — it ends in い, and it inflects exactly like 高い does. (It is the negative-existence adjective; the dedicated ない as an adjective page treats it in full.)

The moment you see that, every further form of the negative comes for free by conjugating ない as an ordinary い-adjective:

• Past: ない → なかった, so 高くない → 高くなかった ("wasn't high")
• て-form: ない → なくて, so 高くない → 高くなくて ("isn't high, and…")

You are not learning new negative endings. You are learning one negative (〜くない) and then applying the plain-adjective rules you already know to the ない half. This is why the past-negative is not a separate memorization job.

いい → よくない (the one irregular)

The adjective いい (good) is irregular in every conjugated form because it swaps its stem to よ- before inflecting. So its negative is よくない, never ×いくない. (Its written form 良い is read either いい or よい, and the よい reading is what surfaces here.)

How this differs from English

English negates an adjective with a separate helper: "is not expensive," "isn't cold." The word not floats in front of the adjective and the verb is carries everything else. Japanese has no floating "not" and no separate "is" — the negation is baked into the adjective's own shape. 高くない is a single inflected word meaning "is-not-expensive," the way an English speaker might imagine "unexpensive" if it inflected. This is why you cannot translate word-for-word: there is no slot for a standalone "not," and hunting for one is what makes learners drop in じゃない (the closest thing to a detachable negator they know). Resist it. The negation belongs inside the adjective, expressed by shifting い to く and appending ない.

Common Mistakes

1. Negating with じゃない. This is the big one. じゃない negates nouns and な-adjectives (学生じゃない, 静かじゃない), never い-adjectives.

2. Forgetting to drop the い before adding くない. You replace the final い, you don't stack onto it.

3. Writing くじゃない — blending the two negatives. Pick one system; the correct piece after く is ない.

4. Saying いくない for いい. いい conjugates on the よ- stem.

Key Takeaways

• Negate an い-adjective by dropping い and adding くない: 高い → 高くない.
• Only the okurigana changes to く; the kanji stem is untouched.
• Polite: 〜くないです (everyday) or 〜くありません (a bit more formal) — both correct.
• Never じゃない with an い-adjective — that pattern belongs to nouns and な-adjectives.
• The ない is itself an い-adjective, so its past (なかった) and て-form (なくて) follow the ordinary rules — the negative is 〜く + ない, not a memorized ending.
• いい is irregular: its negative is よくない, not ×いくない.