Forming Adverbs: 〜く and 〜に | Japanese Grammar

This is the one adverb rule worth more than any list of vocabulary, because it is productive: once you know it, you can turn any adjective in the language into an adverb without ever looking it up. It is Japanese's nearest analogue to English -ly — but where English has a single suffix, Japanese has two, and which one you use is decided entirely by the adjective's class. i-adjectives take 〜く; na-adjectives take 〜に. The deep point of this page is that this split is not an extra thing to memorize: it is the same class distinction you already meet everywhere else in the grammar, wearing adverbial clothes.

The rule in one line

Adjective class	Attributive (before a noun)	Adverb form	Example
i-adjective	〜い (早い電車)	drop 〜い, add 〜く	早い → 早く
na-adjective	〜な (静かな部屋)	add 〜に	静か → 静かに

For an i-adjective, chop the final 〜い and replace it with 〜く:

子供が字を大きく書いた。

kodomo ga ji o ōkiku kaita

The child wrote the letters big.

大きい (big) → 大きく (in a big way). The adverb sits right before the verb it colors, 書く (write).

明日は早く起きなければならない。

ashita wa hayaku okinakereba naranai

I have to get up early tomorrow.

For a na-adjective, don't chop anything — just add 〜に to the stem:

部屋をきれいに掃除した。

heya o kirei ni sōji shita

I cleaned the room thoroughly.

静か (quiet) → 静かに, 簡単（かんたん） (easy) → 簡単に, 上手（じょうず） (skilled) → 上手に:

この問題は簡単に解けた。

kono mondai wa kantan ni toketa

I solved this problem easily.

Why the split is free information

Here is the payoff that makes the rule effortless. The same class distinction that decides に vs く adverbially is the one that decides な vs で attributively and in the copula. You already sort every adjective into i or na to put it in front of a noun. That very sort also hands you the adverb:

If a word takes な before a noun → it is a na-adjective → its adverb takes に. (静かな部屋 → 静かに)
If a word takes plain い before a noun → it is an i-adjective → its adverb takes く. (早い電車 → 早く)

So you never learn "the adverb form" as a separate fact. Identify the adjective class — which you must do anyway — and the adverb form falls out automatically. (If you're shaky on telling the two classes apart, the i-adjective vs na-adjective identification page is the drill.)

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The 〜く / 〜に choice is a reflex of adjective class, not an independent rule. な-word → に; い-word → く. Decide the class once and you know the adverb, the attributive, and the "become" form all at once.

The trap: きれい looks like an i-adjective

The most common derailment isn't the rule — it's misidentifying the class. A handful of na-adjectives end in the kana い and lure you into treating them as i-adjectives: きれい (pretty/clean), 嫌（きら）い (disliked), 有名（ゆうめい） (famous), 丁寧（ていねい） (polite). きれい is the classic trap. Because it's a na-adjective (きれいな花), its adverb is きれいに, never ×きれく.

字をきれいに書いてください。

ji o kirei ni kaite kudasai

Please write the characters neatly.

The tell is the attributive: 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. The final い is part of the stem here, not the i-adjective ending.

The irregular one: いい / よい → よく

There is exactly one irregular you must simply memorize. The adjective いい ("good") is suppletive: whenever it changes form, it reverts to its older stem よ-. So its adverb is not ×いく but よく:

先生の説明はよく分かった。

sensei no setsumei wa yoku wakatta

I understood the teacher's explanation well.

彼は日本語がよくできる。

kare wa nihongo ga yoku dekiru

He can speak Japanese well.

This isn't a special adverb rule — it's the same suppletion that gives いい its negative よくない, its past よかった, and its て-form よくて. Learn いい as "the adjective that turns into よ- the moment it inflects," and よく stops being a surprise. (Note that よく has a second life meaning "often/frequently" — よく映画（えいが）を見る "I often watch movies" — but the core meaning here is "well.")

The same forms feed 〜くなる / 〜になる ("become")

You get a large bonus from this rule. The く and に adverb forms are exactly what the verb なる ("become") attaches to. To say something becomes a certain way, take the adverb form and add なる:

子供はあっという間に大きくなる。

kodomo wa atto iu ma ni ōkiku naru

Children grow up in the blink of an eye.

夜になると、町は静かになる。

yoru ni naru to, machi wa shizuka ni naru

When night falls, the town becomes quiet.

大きい → 大きく → 大きくなる (become big); 静か → 静かに → 静かになる (become quiet). Same く for i-adjectives, same に for na-adjectives — the adverbial split you just learned reappears wholesale in the "become" construction, and later in 〜くする / 〜にする ("make [something] become…"). The full treatment is on the なる (become) page; the point here is that the く/に forms are not a dead end — they are the gateway to a whole family of constructions.

Common mistakes

Mistake 1 — Adding に to an i-adjective. i-adjectives take く, never に. The に belongs to na-adjectives.

❌ 早いに起きる。

Wrong — 早い is an i-adjective, so the adverb is 早く, not 早いに.

✅ 早く起きる。

hayaku okiru

Get up early.

Mistake 2 — Adding く to a na-adjective. na-adjectives take に; there is no ×静かく.

❌ 静かく歩く。

Wrong — 静か is a na-adjective, so the adverb is 静かに, not 静かく.

✅ 静かに歩く。

shizuka ni aruku

Walk quietly.

Mistake 3 — Treating きれい as an i-adjective. きれい ends in the sound い but is a na-adjective, so its adverb is きれいに.

❌ 部屋をきれく掃除した。

Wrong — きれい is a na-adjective (きれいな部屋), so the adverb is きれいに.

✅ 部屋をきれいに掃除した。

heya o kirei ni sōji shita

I cleaned the room thoroughly.

Mistake 4 — Using ×いく for the adverb of いい. いい is suppletive; its adverb is よく.

❌ 彼は日本語がいくできる。

Wrong — いい reverts to the よ- stem when it inflects, so the adverb is よく.

✅ 彼は日本語がよくできる。

kare wa nihongo ga yoku dekiru

He can speak Japanese well.

Key takeaways

i-adjective → 〜く (早い → 早く), na-adjective → 〜に (静か → 静かに). This is Japanese's productive "-ly."
The く/に split is a reflex of adjective class, the same distinction as な/で attributively — identify the class and the adverb is automatic.
きれい, 有名, 嫌い end in the sound い but are na-adjectives → に, not く. Check the attributive (きれいな) to confirm.
いい/よい → よく is the one irregular — the same よ- suppletion behind よくない and よかった.
The く/に forms feed 〜くなる / 〜になる ("become") and 〜くする / 〜にする ("make become") — a whole family, not a dead end.

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