Japanese Adjectives: い and な — The Complete Conjugation Guide

Adjectives That Conjugate

Here's something that surprises English speakers: Japanese adjectives change form. They have past tense. They have negative forms. They conjugate almost like verbs.

In English, "big" is always "big." You use helper words to change meaning: "is big," "was big," "isn't big," "wasn't big." The adjective itself never changes.

In Japanese, the adjective itself transforms:

• 大きい (ōkii) — is big
• 大きかった (ōkikatta) — was big
• 大きくない (ōkikunai) — is not big
• 大きくなかった (ōkikunakatta) — was not big

No helper verbs needed. The adjective does all the work. Once you see the pattern, it's actually more elegant than English.

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Two Types of Adjectives

Japanese has exactly two types of adjectives. Every single adjective in the language falls into one of these groups:

The names come from how they connect to nouns:

• い-adjectives keep their い: 大きい犬 = a big dog
• な-adjectives add な: 静かな部屋 = a quiet room

This い vs な distinction determines everything about how the adjective conjugates. Learn which group an adjective belongs to, and you'll always know what to do with it.

How to Tell Them Apart

い-adjectives always end in い: 高い, 安い, 面白い, 怖い.

な-adjectives usually don't end in い: 静か, 元気, 好き, 暇, にぎやか.

The trap: A few な-adjectives do end in い — and these will fool you:

There's no perfect rule to catch these — you just have to memorize the exceptions. But they're few in number. If an adjective ends in い and it's not in the exceptions list, it's an い-adjective.

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Part 1: い-Adjectives — The Full Conjugation

い-adjectives conjugate by replacing the final い with a new ending. The い is not part of the meaning — it's more like a conjugation ending, similar to how verbs work.

The Four Core Forms

That's the entire system. Four forms, one simple rule: drop い, add the ending.

Making It Polite

To make any form polite, add です (desu) after it:

Unlike verbs (where polite forms change the verb ending), adjective politeness is just "tack on です." The adjective itself conjugates the same way in both plain and polite speech.

Full Example: 面白い (interesting)

More Examples Across Common Adjectives

Every single one follows the same pattern. No exceptions — except one.

The One Irregular い-Adjective: いい (good)

いい is the only い-adjective that doesn't follow the rules. Its stem changes to よ in every form except the plain present:

いい comes from the older form よい (yoi). The present tense shifted to いい over time, but all other forms still use the よ stem. Just memorize it — it's the only one.

かっこいい (cool/handsome) works the same way because it contains いい:

In Practice

昨日は暑かったですね。(Kinō wa atsukatta desu ne.) "It was hot yesterday, wasn't it?"

この映画は面白くなかった。(Kono eiga wa omoshirokunakatta.) "This movie wasn't interesting."

日本語は難しくないです。楽しいです！(Nihongo wa muzukashikunai desu. Tanoshii desu!) "Japanese isn't difficult. It's fun!"

テストはよかったですか？(Tesuto wa yokatta desu ka?) "Was the test good?"

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Part 2: な-Adjectives — A Different System

な-adjectives don't change their own form at all. Instead, they conjugate through the copula — the word that comes after them (だ/です/じゃない, etc.). The adjective itself stays frozen.

Think of it this way:

• い-adjectives are like Japanese verbs — they transform internally
• な-adjectives are like English adjectives — they stay the same while helper words do the work

The Four Core Forms

The adjective 静か never changes. Only the ending (だ → じゃない → だった → じゃなかった) shifts.

Full Example: 元気 (healthy/energetic)

More Examples

Every な-adjective works the same way. No irregular ones.

Formal Alternatives

In more formal speech, じゃ is replaced with では (dewa):

Both are correct. じゃ is the contraction of では. Use じゃ in everyday conversation, では in writing and formal situations.

In Practice

お元気でしたか？(Ogenki deshita ka?) "Have you been well?"

その映画は好きじゃなかった。(Sono eiga wa suki ja nakatta.) "I didn't like that movie."

昨日は暇だったから、買い物に行きました。(Kinō wa hima datta kara, kaimono ni ikimashita.) "I was free yesterday, so I went shopping."

この町は静かじゃないです。にぎやかです。(Kono machi wa shizuka ja nai desu. Nigiyaka desu.) "This town isn't quiet. It's lively."

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Part 3: Modifying Nouns

Both types of adjectives can go directly before nouns to describe them. This is where the な in "な-adjective" finally shows up.

い-Adjectives: Just Place Before the Noun

No changes needed. The adjective keeps its い and sits right before the noun.

な-Adjectives: Add な Before the Noun

The な acts as a connector between the adjective and the noun. Without it, the sentence breaks.

Past and Negative Before Nouns

You can use conjugated adjectives before nouns too. This is where things get expressive:

い-adjectives:

な-adjectives:

Note: for な-adjectives before nouns in past affirmative, you can also use だった directly: 元気だった人 = a person who was healthy.

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Part 4: Two More Useful Forms

The て-Form (Connecting Adjectives)

When you want to chain adjectives — "big and red" — you use the て-form:

い-adjectives: い → くて

大きくて赤い犬 (ōkikute akai inu) "a big and red dog"

安くておいしいレストラン (yasukute oishii resutoran) "a cheap and delicious restaurant"

な-adjectives: add で after the adjective

静かできれいな部屋 (shizuka de kirei na heya) "a quiet and clean room"

元気で優しい先生 (genki de yasashii sensei) "a healthy and kind teacher"

This is the same て-form concept as with verbs — it connects clauses. You can chain as many adjectives as you want.

The Adverb Form

Adjectives can become adverbs — turning "quiet" into "quietly":

い-adjectives: い → く

早く起きました。(Hayaku okimashita.) "I woke up early."

な-adjectives: add に

静かにしてください。(Shizuka ni shite kudasai.) "Please be quiet."

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The Complete Cheat Sheet

い-Adjective Conjugation (example: 楽しい)

な-Adjective Conjugation (example: 好き)

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Side-by-Side Comparison

Here's the clearest way to see how the two types differ:

い-adjectives change internally (い → くない → かった → くなかった). な-adjectives stay the same while the copula changes (だ → じゃない → だった → じゃなかった).

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Common Mistakes to Avoid

1. Conjugating な-adjectives like い-adjectives

❌ 静かくない (shizukakunai) ✅ 静かじゃない (shizuka ja nai)

If it's a な-adjective, it doesn't use くない/かった. Those endings are exclusively for い-adjectives.

2. Treating きれい and 嫌い as い-adjectives

❌ きれくない / 嫌くない ✅ きれいじゃない / 嫌いじゃない

These end in い but they're な-adjectives. きれいな部屋 (not きれい部屋), 嫌いな食べ物 (not 嫌い食べ物). This is the single most common adjective mistake.

3. Forgetting いい conjugates from よ

❌ 天気がいかったです。 ✅ 天気がよかったです。 (Tenki ga yokatta desu.)

いい only appears in the present affirmative. Everything else uses よ: よくない, よかった, よくなかった, よくて, よく.

4. Adding な to い-adjectives before nouns

❌ 面白いな映画 ✅ 面白い映画

い-adjectives connect directly to nouns. Only な-adjectives use な.

5. Saying 高いでした for past tense

❌ 高いでした ✅ 高かったです

い-adjectives conjugate internally — they don't rely on でした for past tense. でした is for な-adjectives (静かでした).

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How to Practice

1. Pick five い-adjectives and five な-adjectives and write out all four forms (present/past × affirmative/negative) for each
2. Describe your day using adjectives: 今日は忙しかったです。天気はよくなかったです。晩ご飯はおいしかったです。
3. Chain adjectives with て-form: Describe a place, person, or thing using two or more adjectives connected with くて/で
4. Play the opposite game: 高い → 安い, 静か → にぎやか, 新しい → 古い, 好き → 嫌い — practice both sides of every pair
5. Spot the type: When you encounter a new adjective, immediately classify it as い or な, then conjugate it in your head

Adjectives let you add color and opinion to your Japanese. Without them, you can state facts. With them, you can say what you think — and thinking out loud is where conversation gets fun.

形容詞をマスターしましょう！ (Keiyōshi o masutā shimashō!) — Let's master adjectives!