Adverbial Form: 〜く / 〜に | Japanese Grammar

To say run quickly or speak quietly — to describe not a thing but how an action is done — you turn an adjective into an adverb. English mostly bolts on "-ly" (quick → quickly). Japanese changes the adjective's ending, and once again the change depends on the class. An i-adjective swaps its 〜い for 〜く (早い → 早く "quickly"). A na-adjective adds 〜に (静か → 静かに "quietly"). This is one of the highest-value forms in the language, because the very same stem also plugs into なる ("become") and する ("make") — so learning the adverb hands you three constructions at once.

i-adjectives: 〜い becomes 〜く

Drop the final い and add く. That adverb then modifies a verb.

早い (fast) → 早く (quickly)
楽しい (fun) → 楽しく (enjoyably)
遅い (slow/late) → 遅く (slowly, late)

明日は早く起きて、始発に乗らないといけない。

ashita wa hayaku okite, shihatsu ni noranai to ikenai

Tomorrow I have to get up early and catch the first train.

子供たちは楽しく遊んでいる。

kodomotachi wa tanoshiku asonde iru

The kids are playing happily.

The く-form attaches directly to the verb — no particle in between. 早く起きる, 楽しく遊ぶ. That directness is the mark of a true adverb.

na-adjectives: add 〜に

A na-adjective takes に on its stem to become an adverb.

静か (quiet) → 静かに (quietly)
簡単 (simple) → 簡単に (easily)
上手 (skilled) → 上手に (skillfully)

図書館では静かに話してください。

toshokan de wa shizuka ni hanashite kudasai

Please speak quietly in the library.

この機械を使えば、簡単に登録できます。

kono kikai o tsukaeba, kantan ni tōroku dekimasu

With this machine you can register easily.

彼女は自転車に上手に乗れる。

kanojo wa jitensha ni jōzu ni noreru

She can ride a bike skillfully.

The に here is the same に-shape as the copula's adverbial form — na-adjectives lean on the copula everywhere, and the adverb is no different. That is why the split is 〜く (i, own stem) vs. 〜に (na, copula stem), exactly parallel to the past (かった/だった) and the te-form (くて/で).

💡

Same question as always: does the adjective inflect on its own stem (い-adj → 〜く) or lean on the copula (な-adj → 〜に)? 早く vs. 静かに. If you know the class, you know the adverb.

The irregular いい → よく — and its double meaning

いい ("good") reverts to its old stem よ whenever it inflects, so its adverb is よく, never ×いく. And よく is a word every learner should treat as special, because it is polysemous: it means both "well" and "often." Context — usually the verb — tells you which.

昨日はよく寝たから、今日は元気だ。

kinō wa yoku neta kara, kyō wa genki da

I slept well last night, so I feel great today.

学生の頃、この喫茶店によく来ました。

gakusei no koro, kono kissaten ni yoku kimashita

Back when I was a student, I often came to this coffee shop.

よく寝た is "slept well" (manner); よく来た is "came often" (frequency). Same word, and only the verb disambiguates. It also means "carefully / thoroughly" in phrases like よく考えて ("think it over carefully"). This is one of the most frequent adverbs in Japanese, so getting comfortable with its range pays off fast.

One stem, three constructions: 〜く/〜に + なる and する

Here is why this form is such a good investment. The exact adverbial stem you just learned also feeds two of the most useful verbs in Japanese, with no extra change:

〜く / 〜に + なる = "become [adjective]" — a natural change of state.
〜く / 〜に + する = "make [something adjective]" / "do [something] in an adjective way" — a deliberate change.

春になって、日が長くなってきた。

haru ni natte, hi ga nagaku natte kita

Spring has come, and the days have gotten longer.

うるさいから、テレビの音を小さくしてくれる?

urusai kara, terebi no oto o chiisaku shite kureru?

It's loud — could you turn the TV down (make it quieter)?

部屋をきれいにしてから出かけた。

heya o kirei ni shite kara dekaketa

I cleaned the room (made it clean) before going out.

長くなる (i-adjective: 長い → 長く + なる), 小さくする (i: 小さい → 小さく + する), きれいにする (na: きれい → きれいに + する). It is the identical stem you use for a plain adverb — 静かにする means both "make it quiet" and "do it quietly." So the moment you can form 静かに and 長く, you have also unlocked "become" and "make." The two verbs get full pages: 〜く/〜になる (become) and 〜く/〜にする (make).

💡

The adverb stem is a three-for-one deal: 早く / 静かに is the adverb, and the front half of 早くなる / 静かになる ("become") and 早くする / 静かにする ("make/do"). Learn one form, use it three ways.

Orthography: only the okurigana changes

A quick but important spelling point. When an i-adjective becomes an adverb, only the trailing kana changes — the kanji stays put. 早い → 早く: the 早 is untouched, only the い→く swap happens in the okurigana. Never rewrite or drop the kanji.

早い → 早く (not ×速い→速う or anything exotic)
新しい → 新しく
忙しい → 忙しく

今週は忙しくて、なかなか連絡できなかった。

konshū wa isogashikute, nakanaka renraku dekinakatta

I was busy this week, so I couldn't get in touch much.

For na-adjectives, the kanji likewise stays and you simply add に in kana: 静か → 静かに, 簡単 → 簡単に.

One kanji trap worth flagging honestly, because it snags even advanced learners: はやい has two kanji with different meanings. 早い is "early" (about time), and 速い is "fast" (about speed) — but both read hayai, so both adverbs are hayaku, written 早く and 速く. You pick the kanji by meaning: 早く起きる ("get up early") uses 早く, while 速く走る ("run fast") uses 速く. There is no pronunciation clue; you simply learn which meaning each kanji carries.

朝は早く起きて、駅までは速く歩く。

asa wa hayaku okite, eki made wa hayaku aruku

I get up early in the morning and walk fast to the station.

In that pair, the first 早く (早い, early) is about time and the second 速く (速い, fast) is about speed — identical sound, different kanji, chosen by meaning.

Common Mistakes

1. Using 〜に with an i-adjective. i-adjectives take 〜く, not に.

❌ もっと早いに走ってください。

motto hayai ni hashitte kudasai

Wrong — 早い is an i-adjective; the adverb is 早く.

✅ もっと早く走ってください。

motto hayaku hashitte kudasai

Please run faster.

2. Using 〜く with a na-adjective. na-adjectives take 〜に.

❌ 静かく歩いてください。

shizukaku aruite kudasai

Wrong — 静か is a na-adjective; the adverb is 静かに.

✅ 静かに歩いてください。

shizuka ni aruite kudasai

Please walk quietly.

3. Forming ×いく from いい. いい's adverb reverts to よく.

❌ 昨日はいく寝た。

kinō wa iku neta

Wrong — いい's adverb is よく.

✅ 昨日はよく寝た。

kinō wa yoku neta

I slept well last night.

4. Slipping an extra を or particle between adverb and verb. The 〜く / 〜に form attaches straight to the verb.

❌ きれいにを書いてください。

kirei ni o kaite kudasai

Wrong — no particle after the adverb; it modifies the verb directly.

✅ きれいに書いてください。

kirei ni kaite kudasai

Please write neatly.

Key Takeaways

To make an adjective describe how an action is done: i-adjectives change 〜い → 〜く (早く走る), na-adjectives add 〜に (静かに歩く). Same class split as every other adjective form.
いい → よく, and よく means both "well" and "often" (and "carefully") — disambiguated by the verb.
The identical stem feeds なる ("become": 長くなる, きれいになる) and する ("make/do": 小さくする, 静かにする) — three constructions from one form.
Orthography: only the okurigana changes (早→早く); the kanji stays. The adverb attaches directly to the verb, with no particle.

Now practice Japanese

Reading grammar gets you part of the way. The exercises are where it sticks — free, no signup needed.

Start learning Japanese→