い-Adjective — Usage & Conjugation

Meaning & Usage

Japanese adjectives split into two groups: い-adjectives (い形容詞, i-keiyoushi) and な-adjectives (な形容詞, na-keiyoushi). This page covers い-adjectives — the group you'll encounter every day from your first lessons onward.

Any adjective whose dictionary form ends in い is an い-adjective. Common examples: 大おおきい (big), 小ちいさい (small), 高たかい (expensive / tall), 安やすい (cheap), 暑あつい (hot), 寒さむい (cold). They serve two roles: modifying nouns directly (attributive use) or acting as the sentence predicate (predicative use).

English adjectives are fixed — "cold" never changes form. Japanese い-adjectives conjugate. They shift to show tense and negation on their own, with no separate verb needed. This is a significant departure from English grammar, and one that catches many learners off guard.

い-adjectives work in both polite and casual speech. In polite sentences, add です at the end. In casual conversation, drop it — the adjective alone is a complete predicate. Note that です here is only a politeness marker; it adds no new meaning to the sentence.

One trap: not every word ending in い is an い-adjective. 綺麗きれい (beautiful) and 嫌きらい (dislike) both end in い but belong to the な-adjective category. When in doubt, confirm the adjective class in a dictionary.

Structure & Formation

Four forms cover everything you need at N5: present affirmative, present negative, past affirmative, and past negative. Each is built by modifying the final い of the adjective.

When an い-adjective modifies a noun, place it directly before the noun in dictionary form. No particle or connector is needed between the adjective and noun.

• Attributive (modifying a noun): い-adjective + Noun → 大おおきい犬いぬ (a big dog)
• Predicative (sentence predicate): Subject + は + い-adjective + です → この犬いぬは大おおきいです。(This dog is big.)

いい (good) is irregular. Its conjugation base shifts to よ: よくない (not good), よかった (was good), よくなかった (was not good). The plain form いい stays as-is; only the conjugated forms use よ.

Example Sentences

Basic Predicative Sentences (Present)

Adjectives Modifying Nouns

Past Tense Forms

The Irregular Adjective いい (Good)

Informal / Plain Speech

Common Mistakes

Mistake 1: Adding だ After an い-Adjective

❌ この本ほんは面白おもしろいだ。

✅ この本ほんは面白おもしろい。

In casual speech, nouns and な-adjectives use だ as the plain copula. い-adjectives already function as complete predicates on their own — adding だ is incorrect. In polite speech use です; in plain speech, the adjective stands alone.

Mistake 2: Conjugating いい Directly Instead of Using よ

❌ 天気てんきはいくなかったです。

✅ 天気てんきは良よくなかったです。

いい is irregular. All negative and past forms use よ as the base: よくない (not good), よかった (was good), よくなかった (was not good). Drill the よ base separately — most beginners trip over this one.

Mistake 3: Treating な-Adjectives Ending in い as い-Adjectives

❌ 部屋へやは綺麗きれいくないです。

✅ 部屋へやは綺麗きれいじゃないです。

Words like 綺麗きれい (beautiful) and 嫌きらい (dislike) look like い-adjectives but are な-adjectives. They use じゃない for negative and だった for past tense. Always confirm the adjective class in a dictionary when in doubt.

Mistake 4: Using the Wrong Form When Modifying a Noun

❌ 高たかかった値段ねだんのレストラン (intended: an expensive restaurant)

✅ 高たかい値段ねだんのレストラン

When an い-adjective modifies a noun in present context, use the dictionary form (ending in い). The past form かった belongs in predicate position — not in front of a noun you're describing right now.

Mistake 5: Forgetting to Drop い Before Conjugating

❌ 高たかいくない / 高たかいかった ✅ 高たかくない / 高たかかった

Always remove the final い before adding a conjugation suffix. The pattern is: stem + suffix (くない, かった, くなかった). Leaving the い in place is a very common early error.

Cultural Notes

Native speakers often drop the subject entirely and let the adjective carry the moment. After a good meal, saying just 美味おいしい！ is perfectly natural — no subject, no verb needed. Short exclamatory い-adjectives like this come up constantly in casual conversation.

Negative opinions are often softened through past tense or shared observation. 高たかかったですね (That was a bit pricey, huh) lands more gently than 高たかすぎます (It's too expensive). Japanese conversation tends to favor this kind of indirect framing, especially when expressing criticism.

Weather talk is a go-to conversation opener in Japan. 寒さむいですね or 暑あついですね — paired with ね — function almost like greetings. They invite agreement and ease into small talk naturally.

In advertising, menus, and product packaging, plain-form い-adjectives are everywhere. Phrases like 美味おいしい料理りょうり (delicious cuisine) or 新あたらしいデザイン (new design) appear on signs and labels throughout Japan.

Related Grammar Points

• Na-Adjective (な形容詞) — Complete Usage Guide (Grammar N5)
• Negative I-Adjective: くない (ku nai) (Grammar N5)
• Past Tense of I-Adjectives — かった (Grammar N5)
• たい — Want To Do (Grammar N5)
• ない — Negative Form (Not) (Grammar N5)
• の — Possessive & Noun Modifier Particle (Grammar N5)

JLPT Tips

い-adjective conjugation is a sure thing on the N5 exam. Expect questions asking you to pick the correct form for a given tense and polarity. The past negative form (くなかった / くなかったです) is the trickiest of the four and comes up often in fill-in-the-blank questions.

The いい → よ shift will almost certainly appear on the exam. Wrong choices like いくない or いかった are deliberate traps. When a sentence calls for a conjugated form of いい, the base is always よ — no exceptions.

In reading passages at N5, い-adjectives often appear in plain form modifying nouns. Practice recognizing phrases like 大おおきい声こえ (loud voice) or 新あたらしい学校がっこう (new school) quickly, without pausing to analyze.

For listening, know that in fast speech くない and くなかった can sound very similar. Focus on the final syllable: い for present negative, た for past negative. Audio questions often test exactly this distinction.

Group い-adjectives into antonym pairs when you study: 大おおきい/小ちいさい (big/small), 高たかい/安やすい (expensive/cheap), 長ながい/短みじかい (long/short), 速はやい/遅おそい (fast/slow). You pick up twice the vocabulary at once, and conjugating them together makes the patterns stick faster.