Meaning & Usage
Negating an い-adjective in Japanese follows a single rule: drop the final い and add くない. 高い (expensive, tall) becomes 高くない. 面白い (interesting) becomes 面白くない. The same pattern works for nearly every い-adjective in the language.
English negates adjectives with "not" placed before them — "not tall," "not interesting." Japanese instead changes the adjective's own ending. This is why い-adjectives are sometimes compared to verbs: they conjugate to show tense, negativity, and how they attach to surrounding clauses.
くない is the casual everyday form — what you hear among friends and family. In formal situations, くないです suits most daily contexts: school, work, or someone you've just met. くありません appears in formal writing and official speech. All three mean the same thing at different registers. For N5, くない and くないです cover most real conversations.
Think of い-adjectives as self-conjugating words. Like verbs, they shift their endings to show tense and mood. The negative always ends in くない, regardless of which adjective you're using. Once that pattern is internalized, the negative form comes without conscious effort.
くない works as both a predicate and a pre-noun modifier. 高くない stands alone as a complete statement — "it is not expensive." 高くない本 modifies a noun directly — "a book that is not expensive." That range makes it useful across many different sentence structures.
Structure & Formation
The formation rule is consistent across all い-adjectives. Find the dictionary form, remove the final い, and attach くない to the remaining stem.
| Dictionary Form | Stem (drop い) | Negative Form | Meaning |
|---|---|---|---|
| 高い | 高 | 高くない | not expensive / not tall |
| 大きい | 大き | 大きくない | not big |
| 小さい | 小さ | 小さくない | not small |
| 寒い | 寒 | 寒くない | not cold |
| 面白い | 面白 | 面白くない | not interesting |
| 難しい | 難し | 難しくない | not difficult |
Critical exception: いい (good)
The adjective いい (good) is irregular. Its negative form is 良くない, not いくない. This is because いい historically derives from よい, and all conjugated forms use よ as the base stem. This is the single most important exception to memorize at the N5 level.
Polite and formal variations:
- Casual: [stem] + くない — used with friends and family
- Polite: [stem] + くないです — used in most daily formal situations
- Formal: [stem] + くありません — used in writing, business, official speech
Example Sentences
Basic Everyday Descriptions
この本は高くない。
Kono hon wa takakunai.
This book is not expensive.
あの山は大きくない。
Ano yama wa ookikunai.
That mountain is not big.
この部屋は小さくない。
Kono heya wa chiisakunai.
This room is not small.
Weather and the Environment
今日は寒くない。
Kyou wa samukunai.
Today is not cold.
今日の天気は悪くない。
Kyou no tenki wa warukunai.
Today's weather is not bad.
夏はそんなに暑くない。
Natsu wa sonna ni atsukunai.
Summer is not that hot.
Feelings and Personal States
あまり忙しくない。
Amari isogashikunai.
I'm not very busy.
この映画は面白くない。
Kono eiga wa omoshirokunai.
This movie is not interesting.
この料理は辛くない。
Kono ryouri wa karakunai.
This dish is not spicy.
Polite Form in Context
この問題は難しくないです。
Kono mondai wa muzukashikunai desu.
This problem is not difficult.
そのかばんは重くないです。
Sono kaban wa omokunai desu.
That bag is not heavy.
The Irregular いい → よくない
この考えは良くない。
Kono kangae wa yokunai.
This idea is not good.
今日は体の調子が良くない。
Kyou wa karada no choushi ga yokunai.
I'm not feeling well today.
その答えは良くないです。
Sono kotae wa yokunai desu.
That answer is not good.
Common Mistakes
Mistake 1: Using くない with な-adjectives
❌ 静かくない ✅ 静かじゃない
Beginners make this mistake often. くない is exclusively for い-adjectives. な-adjectives such as 静か (quiet), 元気 (healthy/energetic), and 便利 (convenient) use じゃない or ではない for negation. The key distinction: い-adjectives end in the hiragana い in their dictionary form, while な-adjectives typically do not, and they require な when modifying a noun directly.
Mistake 2: Negating いい as いくない
❌ この映画はいくない。
✅ この映画は良くない。
The adjective いい is completely irregular. All conjugated forms use the stem よ, not い — so the negative is よくない, the past tense is よかった, and the negative past is よくなかった. The form いくない does not exist. Always remember: いい only ever appears as the dictionary form; every conjugation builds on よ.
Mistake 3: Forgetting to drop い before adding くない
❌ 大きいくない ✅ 大きくない
Some learners attach くない without first removing the final い, producing forms like おおきいくない or むずかしいくない. The rule is non-negotiable: strip the い first, then attach くない to the bare stem. This slip is common early on and fades quickly with practice.
Mistake 4: Using くない instead of くなかった for past tense
❌ 昨日は寒くない。(intending: Yesterday was not cold)
✅ 昨日は寒くなかった。
くない covers the present or non-past negative — how things are now or in general. For past states, use くなかった. Mixing these up is natural at first. The shortcut: くない = "is not" (now), くなかった = "was not" (then).
Mistake 5: Dropping the く entirely
❌ この映画は面白ない。
✅ この映画は面白くない。
Dropping the く and saying おもしろない sounds incomplete to native speakers. The く is not optional — the full negative ending is always くない, never bare ない on its own. Omitting it usually means the pattern hasn't fully clicked yet.
Cultural Notes
くない comes up constantly in casual Japanese — any informal conversation will have it multiple times. Native speakers often pair it with softening adverbs: あまり (not very), そんなに (not that much), or ぜんぜん (not at all). あまり高くない — "not very expensive" — sounds measured and considerate rather than bluntly negative.
Directness can feel abrasive in Japanese social contexts. Rather than calling something bad (わるい) outright, people often reach for "not good" (良くない) instead. This shows up when giving feedback, expressing mild dissatisfaction, or redirecting someone — the meaning lands clearly, but the edge is gone.
Among younger speakers, くない at the end of a sentence with rising intonation — くない？ — acts as a tag question, roughly "isn't it?" or "right?" Context can flip the meaning entirely. この映画、面白くない？ can mean "This movie isn't interesting, is it?" or "This movie is great, don't you think?" — tone and situation decide which. The usage is casual and distinctly contemporary.
In informal digital writing — texts, social media — の sometimes follows くない to add a softer, more explanatory feel: 高くないの。(It's not expensive, you know.) The の invites the listener into the speaker's perspective, which sounds warm in close relationships.
Related Grammar Points
- い-Adjective — Usage & Conjugation (Grammar N5)
- Na-Adjective (な形容詞) — Complete Usage Guide (Grammar N5)
- Past Tense of I-Adjectives — かった (Grammar N5)
- じゃない — Negative Form for Nouns and Na-Adjectives (Grammar N5)
- たい — Want To Do (Grammar N5)
- しか — Nothing But, Only (Negative) (Grammar N5)
JLPT Tips
On the N5 exam, くない appears most often in grammar fill-in-the-blank and short reading passages. You'll need to produce the correct negative form for common い-adjectives and distinguish them from the negative forms of nouns and な-adjectives.
When you hit a negation question: strip the final い to find the stem, attach くない. Drill this with the highest-frequency N5 adjectives: 大きい, 小さい, 高い, 安い, 新しい, 古い, 難しい, 面白い, 暑い, 寒い, and いい. The goal is instant recall — no hesitation on exam day.
Watch for the い-adjective vs な-adjective trap. A classic question presents a な-adjective like 元気 or 好き and asks for the correct negative, with both くない and じゃない as options. Always identify the adjective type first: if it ends in い as an い-adjective, use くない; if it's a な-adjective or noun, use じゃない.
Review the full conjugation paradigm: positive present (高い), negative present (高くない), positive past (高かった), negative past (高くなかった). The N5 can test any of these four forms — knowing all of them means no conjugation question catches you off guard.
The listening section adds another challenge: catching くない at natural speech speed, where sounds blend and syllables get swallowed. Watching beginner-aimed Japanese content — slow-paced news, simple anime, study channels — builds the ear recognition you need before exam day.