4.7 The Adjective System: い/な Conjugation, Adverbial 〜く/〜に & Degree Modifiers

The Adjective System: い-Adjectives, な-Adjectives and Their Forms

Japanese adjectives split into two classes that behave like different parts of speech. い-adjectives (形容詞) conjugate on their own, like verbs. な-adjectives (形容動詞) do not conjugate at all — they lean on the copula. Almost every N4 grammar pattern in Chapters 5 and 6 attaches to one of the forms below, so an error here propagates everywhere.

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1. The Two Paradigms Side by Side

い-Adjectives: 高い (expensive)

The rule is uniform: drop い, add the ending. Note that the polite present is 高いです — the です is bolted on and does not replace い. ✗高です is wrong.

な-Adjectives: 静か (quiet)

な-adjectives are really nouns wearing an adjective's job: everything above is the copula だ conjugating, not the adjective.

The Two Positions That Trip Learners

✗静かに部屋 and ✗静かな話します are both standard Mondai 1 distractors. The mnemonic: な touches a noun, に touches a verb.

The Ones That Look Like the Wrong Class

Two な-adjectives end in the letter い and are therefore mis-conjugated constantly:

• きれい(な) — pretty, clean
• 嫌い(な) — disliked

So it is 「きれいな花」 and 「きれいじゃない」, never ✗きれくない; and 「嫌いな食べ物」, never ✗嫌くない. The giveaway is that their い is part of the kanji reading (綺麗 / 嫌い), not a conjugating ending.

The rest of the high-frequency N4 な-adjectives do not end in い at all, but are worth holding as a set because every one of them needs な before a noun and に before a verb: 有名(な) (famous), 元気(な) (well), 親切(な) (kind), 便利(な) (convenient), 上手(な) (skilled), 下手(な) (unskilled), 大切(な) (important), 簡単(な) (easy), 大変(な) (tough), 特別(な) (special), 安全(な) (safe), 不便(な) (inconvenient).

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2. いい / よい: The One Irregular Adjective

いい is regular only in the plain present. Everywhere else it reverts to its older stem よ-:

Every compound built on いい inherits the irregularity: 格好いい → 格好よかった, 気持ちいい → 気持ちよかった.

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3. The Adverbial Forms and What Needs Them

The 〜く and 〜に forms are not decoration — three of the highest-frequency N4 patterns are built on them.

The なる/する contrast is the whole of Section 6.3, but it depends entirely on producing 大きく and 静かに correctly first. ✗大きいになる and ✗静かくなる are the two errors that recur.

The adverbial form is also how you build ordinary adverbs from adjectives: 早く起きる (get up early), 上手に話す (speak skilfully), 簡単に説明する (explain simply).

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4. Attachment: Stem or Plain Form?

Different suffixes grab different parts of the adjective, and Mondai 1 tests the boundary directly.

The single most tested contrast in this table: 高そうです (it looks expensive — you are looking at it) versus 高いそうです (I hear it's expensive — someone told you). One い decides which. For な-adjectives and nouns, hearsay inserts だ: 便利だそうです, 病気だそうです — while visual conjecture does not: 便利そうです.

Two irregular stems to memorise: いい → よさそう and ない → なさそう.

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5. Comparatives and Degree Adverbs

The comparative machinery is covered in Section 6.2; what belongs here is the degree adverbs that scale an adjective, because Mondai 3 tests their polarity restrictions:

✗「あまり高いです」 is ungrammatical — あまり and 全然 are bound negative-polarity adverbs and must find a negative predicate, exactly like しか in Section 4.5.

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6. Adjectives Built From Verbs: 〜やすい, 〜にくい and 〜方

Section 4 established that 〜すぎる attaches to a bare stem. Three more N4 suffixes use the same masu-stem slot, and two of them manufacture a brand-new い-adjective — which means everything in Sections 1 and 3 then applies to the result.

The step learners skip. 〜やすい and 〜にくい are not frozen endings — the word they build is a full い-adjective and must go on conjugating:

• negative: この字は読みにくくないです。
• past: 新しい薬は飲みやすかったです。
• te-form linking: このペンは書きやすくて、安いです。
• adverbial: 分かりやすく説明してください。

✗「読みにくいでした」 and ✗「読みにくくないでした」 are the two forms Mondai 1 rewards you for rejecting — an い-adjective carries its own past and negative, exactly as in Section 1.

〜方 makes a noun, not an adjective, so it takes particles like any other noun, and the object particle in front of it shifts from を to の:

• ✓ この漢字の読み方を教えてください。 (Please teach me how to read this kanji.)
• ✗ この漢字を読み方を教えてください。

The Mondai 1 pair trap. やすい and にくい are near-mirror distractors, and the item is decided by the surrounding evaluation, not by the verb: 「この靴は軽いので、とても歩き（　）です。」 takes やすい because 軽い is positive; 「この道は狭くて暗いので、夜は歩き（　）です。」 takes にくい because 狭い・暗い are negative. Read the adjective in the reason clause before you choose the suffix.