English is generous: many words double as both adjective and adverb with no change of shape. A fast car (adjective, before a noun) and eat fast (adverb, after a verb) reuse the identical word fast. Japanese refuses that. Every adjective has two distinct outfits — an attributive form for modifying nouns (早い電車 "a fast train") and an adverbial form for modifying verbs (早く着く "arrive early") — and you cannot wear one to the other's job. The signature error is leaving an adjective in its noun-modifying い or な shape when a verb is coming next: ×早い食べる, ×静かな話す. The fix is to convert it. This page drills that conversion as an error-correction; the productive rule is taught on Forming Adverbs: 〜く and 〜に.
The one diagnostic: is a verb coming next?
Before you use an adjective, look at what it modifies. That single check tells you which outfit to wear:
- Modifying a noun (電車, 部屋) → attributive: い-adjective keeps 〜い (早い電車), na-adjective takes 〜な (静かな部屋).
- Modifying a verb (食べる, 話す) → adverbial: i-adjective becomes 〜く (早く), na-adjective takes 〜に (静かに).
早い電車に乗って、早く着いた。
hayai densha ni notte, hayaku tsuita
I took an early train and arrived early.
Both 早い電車 (before a noun) and 早く着いた (before a verb) come from the same adjective 早い — but the outfit switches with the job. English uses early twice unchanged; Japanese uses 早い then 早く. Seeing a verb ahead is your cue to convert.
i-adjectives: drop 〜い, add 〜く
To modify a verb with an i-adjective, chop the final 〜い and replace it with 〜く. Leaving the 〜い on is the direct calque of English "eat fast."
時間がないから、もっと早く歩こう。
jikan ga nai kara, motto hayaku arukō
We're short on time, so let's walk faster.
名前は大きく書いてください。
namae wa ōkiku kaite kudasai
Please write your name big.
みんなで楽しく遊んだ。
minna de tanoshiku asonda
We all had fun playing together.
早い → 早く, 大きい → 大きく, 楽しい → 楽しく. The one irregular is いい ("good"), which reverts to its old よ- stem: its adverb is よく ("well"), never ×いく — the same suppletion behind よくない and よかった.
na-adjectives and nouns: add に
na-adjectives don't chop anything — you add 〜に to the stem. Two errors happen here: using the attributive 〜な before a verb (×静かな話す), or leaving the bare stem with no particle at all (×静か話す). Both need 〜に.
赤ちゃんが寝ているから、静かに歩いてね。
akachan ga nete iru kara, shizuka ni aruite ne
The baby's asleep, so walk quietly, okay?
彼女は日本語をとても上手に話す。
kanojo wa nihongo o totemo jōzu ni hanasu
She speaks Japanese very skillfully.
この機械は簡単に使えるよ。
kono kikai wa kantan ni tsukaeru yo
This machine is easy to use.
静か → 静かに, 上手 → 上手に, 簡単 → 簡単に. This に is also how plain nouns become adverbs — 本当 → 本当に ("really"), 別 → 別に ("separately") — so treat "na-adjective/noun → adverb" as one rule: add に.
The きれい trap
The most common derailment isn't the rule, it's misjudging the class. A handful of na-adjectives end in the kana い and masquerade as i-adjectives: きれい (pretty/clean), 嫌い (disliked), 有名 (famous), 丁寧 (polite). きれい is the classic. Because it's a na-adjective (きれいな花), its adverb is きれいに — never ×きれく or ×きれいく.
お皿をきれいに洗ってから、棚に戻してね。
o-sara o kirei ni aratte kara, tana ni modoshite ne
Wash the plates clean, then put them back on the shelf.
The check that saves you: what does it look like before a noun? You say きれいな人, not ×きれい人 — so it's a na-adjective, so the adverb is きれいに. That final い is part of the stem, not the i-adjective ending. If telling the two classes apart is the shaky part, drill it on i-adjective vs na-adjective identification.
The payoff: く/に feed なる and する
Converting to the adverbial form is not a dead end you use only before verbs — those exact く and に forms are what the "become / make" verbs attach to. なる ("become") and する ("make [it become]") take them directly:
最近、野菜が高くなったね。
saikin, yasai ga takaku natta ne
Vegetables have gotten expensive lately, haven't they.
図書館では静かにしなさい。
toshokan de wa shizuka ni shinasai
Be quiet in the library. (make yourself quiet)
高い → 高く → 高くなる ("become expensive"); 静か → 静かに → 静かにする ("make it quiet / be quiet"). Same く for i-adjectives, same に for na-adjectives — the adverbial split reappears wholesale here. So the conversion you're drilling to fix this error also unlocks a whole family of constructions; the full treatment is on なる (become).
Common mistakes
1. Bare 〜い before a verb. i-adjectives take 〜く to modify a verb, not the attributive 〜い.
❌ もっと早い歩いて。
motto hayai aruite
Wrong — before a verb, 早い becomes 早く: もっと早く歩いて.
✅ もっと早く歩いて。
motto hayaku aruite
Walk faster.
2. 〜な before a verb. The attributive 〜な modifies nouns; before a verb, use 〜に.
❌ 静かな話してください。
shizuka na hanashite kudasai
Wrong — 〜な is for nouns; before a verb it's 静かに: 静かに話してください.
✅ 静かに話してください。
shizuka ni hanashite kudasai
Please speak quietly.
3. Bare na-adjective stem with no に. A na-adjective can't sit naked before a verb; it needs に.
❌ ピアノを上手弾きたい。
piano o jōzu hikitai
Wrong — the stem needs に: 上手に弾きたい.
✅ ピアノを上手に弾きたい。
piano o jōzu ni hikitai
I want to play the piano skillfully.
4. Treating きれい as an i-adjective. きれい is a na-adjective, so the adverb is きれいに, not ×きれく.
❌ 部屋をきれく片付けた。
heya o kireku katazuketa
Wrong — きれい is a na-adjective (きれいな部屋), so the adverb is きれいに.
✅ 部屋をきれいに片付けた。
heya o kirei ni katazuketa
I tidied the room up neatly.
5. ×いく for the adverb of いい. いい is suppletive; its adverb is よく.
❌ 前より、いく眠れるようになった。
mae yori, iku nemureru yō ni natta
Wrong — いい's adverb is よく: よく眠れるようになった.
✅ 前より、よく眠れるようになった。
mae yori, yoku nemureru yō ni natta
I've come to sleep better than before.
Key takeaways
- Every adjective has two outfits: attributive (before a noun — 早い電車 / 静かな部屋) and adverbial (before a verb — 早く / 静かに). English reuses one word; Japanese doesn't.
- Diagnostic: a verb coming next means you need the adverbial form. A bare 〜い or 〜な in front of a verb is the error.
- i-adjective → 〜く (早い → 早く); na-adjective / noun → 〜に (静か → 静かに, 本当 → 本当に).
- きれい, 有名, 嫌い end in the sound い but are na-adjectives → に. Check the attributive (きれいな) to confirm.
- The same く/に forms feed 〜くなる / 〜になる ("become") and 〜にする — the conversion is a gateway, not a chore.
Now practice Japanese
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Start learning Japanese→Related Topics
- Adverbial Form: 〜く / 〜にN4 — Turning adjectives into adverbs — i-adjectives change 〜い to 〜く (早く走る), na-adjectives add 〜に (静かに歩く) — the same stem that also feeds なる 'become' and する 'make', plus the よく polysemy.
- i-Adjectives: PresentN5 — The dictionary form of an い-adjective ends in the kana い and works two ways with no helper word — straight before a noun (面白い本) and as a complete predicate ending a sentence (この本は面白い) — because the adjective already contains its own 'to be.'
- na-Adjectives: PresentN5 — How な-adjectives predicate in the present — they behave like nouns and borrow the copula だ/です rather than predicating on their own.