Adverbial Forms of Japanese Adjectives: く and に

The adverbial forms of Japanese adjectives let an adjective describe a verb instead of a noun. An い-adjective swaps its final い for く (早い → 早く), and a な-adjective attaches に directly to its base (静か → 静かに).¹²³ At JLPT N5, this is the rule that turns "quick" into "quickly" and "quiet" into "quietly." It also unlocks the workhorse なる / する patterns that every introductory textbook leans on.⁴⁵⁶

Overview

The adverbial form is one cell in the same six-base inflection table that produces an adjective's negative, te-form, and past tense.¹ It feels hard when treated as a separate rule. It becomes easier when treated as one row of a table the learner has already met.

Where this article sits in the adjectives map

The sibling article "Japanese Adjectives Overview: The Two Classes (い-形容詞 vs な-形容詞)" introduces the two productive adjective classes. The full inflection tables for each class live in "い-Adjective Conjugation in Japanese: All Tenses and Forms" and "な-Adjective Conjugation in Japanese: All Tenses and Forms".¹ The adverbial form is one row in each of those tables.

The sibling cell that shares the same stem is the topic of "Adjective Te-Form in Japanese: How to Link with くて and で". Both cells branch from the same く-stem on the い side and from the same copula 連用形 (renyōkei, or continuative form) on the な side.¹² The parallel renyōkei concept on the verb side is the topic of "Verb Stem (連用形): The Most Productive Form in Japanese", which gives the broader picture of why this cell is so productive across the language.¹

The one-sentence rule

An い-adjective becomes its adverbial form by replacing the final い with く. A な-adjective becomes its adverbial form by attaching に directly to the base. The resulting word sits in front of a verb and describes how the action happens.¹²³

The same form also feeds two support patterns the learner will meet on every N5 page: ～なる ("become") and ～する ("make").¹⁴

早はやく起おきました。⁷⁸
"I got up early."

静しずかに歩あるいた。⁹
"I walked quietly."

Why this is the same paradigm, not two unrelated rules

The ～く suffix on an い-adjective is the 連用形 (continuative / adverbial form) of the adjective itself. The ～に suffix on a な-adjective is the 連用形 of the copula だ.¹²³ Japanese uses two surface shapes for the same cell because the two classes inflect through different paradigms, not because the rule itself is split.

In one line: く is the い-class adverbial stem (the same stem that appears in the negative 大きくない and the te-form 大きくて). に is the な-class adverbial cell of the copula paradigm (the same に that appears in 学生になる).¹¹⁰²³ Seeing them as parallel cells in one table helps prevent the most common N5 error in this paradigm.

The diagram shows the structural symmetry: two surface shapes (く and に) feed the same three downstream uses (manner of a verb, ～なる, ～する).¹²³⁴

The に here is not the destination particle

The に of 静かに歩く is the adverbial cell of the copula だ, the same cell that surfaces in 学生になる ("become a student").¹⁰³ It is unrelated to the に that marks destination in 学校に行く or time in 三時に始まる.

Formation

い-adjectives: drop い, add く

Replace the final い with く: 早い → 早く, 大きい → 大きく, おいしい → おいしく.¹⁸ The く form is the bare 連用形 stem. No particle or suffix attaches after it to mark the adverbial role; it chains directly into the verb it modifies.¹²⁸

The same く-stem feeds two other cells the reader already knows: the te-form 早くて and the negative 早くない.¹² One stem, three suffixes.

Dictionary	く-stem (adverbial)	Te-form	Negative
早はやい	早はやく	早はやくて	早はやくない
大おおきい	大おおきく	大おおきくて	大おおきくない
おいしい	おいしく	おいしくて	おいしくない

子供達こどもたちは大おおきくなっていた。⁸
"The kids had gotten big."

そのラーメンを美味おいしく食たべました。⁸
"I enjoyed that ramen."

部屋へやを明あかるくしました。⁸
"I made the room bright."

For the full cell map (six bases, with polite / plain and affirmative / negative forms), see the sibling article "い-Adjective Conjugation in Japanese: All Tenses and Forms".¹

The いい / 良い irregular: よく, not いく

The dictionary form いい / 良い is the one い-adjective with an irregular adverbial form. Every cell except the present and the attributive is built on the older よ-stem, so the adverbial form is よく.¹¹ There is no ✗ いく or ✗ いかった.¹¹

The pattern is the same one the reader has met in the past よかった, the negative よくない, and the te-form よくて: every non-present cell uses よ-, never い-.¹¹ Historically, modern いい is a casual sound shift from older よい / 良し. The older stem survived in every cell except the one where the shift happened.¹¹

よく分わかりました。¹¹
"I understand well."

よく寝ねました。¹²
"I slept well."

よく carries two adverbial senses at once

The same form よく covers both "well / properly" and "frequently / often." Native speakers parse them by context: よく寝る usually reads "sleep well," and よく見る usually reads "see often."¹² Both senses descend from the same root よし / よい. In modern Japanese, they behave like two adverbs sharing a form.¹²

For the full paradigm and the 良し → 良い merger, see the sibling article "The いい / 良い Irregular: Why the Past Is よかった".¹¹

な-adjectives: base + に

Drop な (or use the bare base) and attach に: 静か → 静かに, きれい → きれいに, 上手 → 上手に.¹¹⁰³ The に here is the 連用形 cell of the copula だ, not a particle. In linguistic terms, "the ～く morpheme has a function analogous to the に adverbial copula, which would be the renyōkei of the だ copula used with な-adjectives and の-adjectives."²

The same に surfaces in noun + に + なる (学生になる, "become a student") and in adverbialized noun phrases like 普通に ("normally") and 自由に ("freely"); all of these are the same copular 連用形.¹¹⁰³

Dictionary base	Attributive (before noun)	Adverbial (before verb)
静しずか	静しずかな	静しずかに
きれい	きれいな	きれいに
上手じょうず	上手じょうずな	上手じょうずに
便利べんり	便利べんりな	便利べんりに

私わたしは静しずかにドアを閉しめた。⁹
"I closed the door quietly."

バラが綺麗きれいに咲さいていた。⁹
"The roses were blooming beautifully."

静しずかになりました。³⁴
"It became quiet."

The historical anchor is short: な-adjectives are the modern reflex of the Classical Japanese nari-adjectives. In that older pattern, ni aru contracted to nari, whose attributive naru later contracted to な.¹⁰ The に cell survived from before the contraction. な and に are not arbitrary endings, but two cells of one copula paradigm.¹⁰³

For the full cell map, see "な-Adjective Conjugation in Japanese: All Tenses and Forms"; for the broader copula picture, "The Japanese Copula: です, だ, である Explained".¹

The "do not use く with な-adjectives" trap

The single most common N5 error in this paradigm is overgeneralizing the い-class rule to な-adjectives: ✗ 静かく, ✗ きれいく.¹³ Both are documented as a recurring error class on JLPT reference pages.¹³

Class decides the suffix, not the surface sound

きれい ends in the syllable い but is a な-adjective (its attributive is きれいな部屋, not ✗ きれい部屋). The adverbial form is therefore きれいに, never ✗ きれいく.¹¹³ The diagnostic is the attributive: if the word needs な before a noun, it takes に before a verb.

静しずかに歩あるいた。⁹¹³
"I walked quietly."

The diagnostic the reader already knows is the attributive pattern: an い-adjective never needs な before a noun (大きい部屋), and a な-adjective always does (静かな部屋).¹¹³ The adverbial cell follows the same class boundary. い-adjectives take く, な-adjectives take に, and no class crosses to the other suffix.

The sibling article "Attributive vs. Predicative Adjectives in Japanese: Why 静か Needs な But 大きい Does Not" maps this position-by-position.

Negative adverbial: なく and ではなく

The negative chain has its own adverbial cell. For an い-adjective, the negative form 大きくない itself behaves like an い-adjective and applies the い → く rule, giving 大きくなく ("not big" in adverbial position).¹ For a な-adjective or a noun, the negative is ではない, and its adverbial cell is ではなく / じゃなく.¹⁴

Register matters. The bare なく / ではなく forms are the 連用形 of the negative and read as formal written connectors, often in the contrastive pattern "X ではなく Y" ("not X, but Y").¹⁴ The spoken default for the same content is the te-form なくて / ではなくて.¹⁴

このスープは甘あまくなく、辛からくもない。¹¹⁴
"This soup is neither sweet nor spicy."

問題もんだいは予算よさんではなく、時間じかんです。¹⁴
"The problem is not the budget, but time."

At N5, なく and ではなく are recognition-level forms. The reader does not need to produce them actively yet, only parse them when they appear in writing.¹⁴

Nuance and usage contexts

Modifying a verb of action: the default use

The most direct use of the adverbial form is manner of action, meaning how the verb happens.²⁸ The form sits directly before the verb it modifies, not at the end of the clause.⁸⁹

彼かれらは楽たのしく暮くらしていました。⁸
"They were living happily."

私わたしは京都きょうとに長ながく住すんでいました。⁸
"I lived in Kyoto for a long time."

バラが綺麗きれいに咲さいていた。⁹
"The roses were blooming beautifully."

The same syntactic slot also accepts lexical adverbs that do not derive from adjectives (とても, ちょっと, すぐ). The two types can stack, with the lexical intensifier before the derived adverb (とても早く走る, "run very fast").¹

～なる: the intransitive change of state

The first support pattern is the adverbial form plus なる. It expresses that the subject comes to be in the state the adjective names.⁴ The construction is intransitive: the subject changes, with no agent required.⁴

For an い-adjective, the rule is "delete い, add く," then attach なる: 大きい → 大きくなる ("become big / grow up").⁴ For a な-adjective, the rule is "drop な, add に + なる": 静か → 静かになる ("become quiet").⁴ The class only decides whether the connector is く or に. なる itself is the same verb in both.

寒さむくなりました。¹
"It has gotten cold."

静しずかになりました。⁴
"It became quiet."

もう少すこし安やすくなりませんか。⁴
"Could it be made a little cheaper?"

The parallel pattern noun + に + なる (学生になる, "become a student") uses the same に as 静かになる.¹³ One copular に bridges the adjective side and the noun side of the language. That is why "become quiet" and "become a student" sound structurally identical in Japanese.

～する: the transitive causation

The second support pattern is the adverbial form plus する. It expresses that an agent makes a patient enter the state the adjective names.⁴ The diagnostic against なる is the argument structure: する takes a を-marked patient and an (often implicit) agent; なる takes neither.⁴

For an い-adjective: 大きい → 大きくする ("make X bigger").⁴ For a な-adjective: 静か → 静かにする ("make / keep X quiet").⁴

スープを甘あまくした。⁴
"I made the soup sweet."

部屋へやを明あかるくしました。⁸
"I made the room brighter."

静しずかにしてください。³
"Please be quiet."

The polite request 静かにしてください is the form learners encounter most often in classrooms, libraries, and public announcements. The literal reading is "please make yourself quiet."³ The parallel noun + に + する construction (息子を医者にする, "make my son a doctor") reuses the same copular に once again.³

The diagram shows the なる / する split as one pair: same adverbial input, two argument frames.⁴

Speech verbs: はっきり言う, 大きく言う, 詳しく説明する

Speech and assertion verbs (言う "say," 話す "speak," 書く "write," 説明する "explain") take adverbial modifiers freely. The adverbial form occupies the same pre-verbal slot as before.⁸ Some modifiers in this slot are derived adverbs (大きく, 静かに, 詳しく). Others are lexical adverbs that do not derive from adjectives (はっきり, ゆっくり), but they share the same syntactic position.¹

静しずかに話はなしました。³⁹
"I spoke softly."

大おおきく書かきました。⁸
"I wrote in large letters."

詳くわしく説明せつめいしてください。⁸
"Please explain in detail."

The set phrases 簡単に言うと ("to put it simply") and 詳しく言うと ("to put it in detail") are common conversational lead-ins. Both depend on the adverbial form sitting before the speech verb.⁸

Verbs of motion: 走る, 歩く, and friends

Motion verbs commonly combine with manner and speed adverbs. Both い-class adverbs (早く, 速く, 大きく) and な-class adverbs (静かに) appear in this slot.⁸⁹

早はやく走はしった。⁷⁸
"I ran fast."

速はやく返信へんしんしてくれた。¹⁵¹⁶
"They replied fast."

静しずかに歩あるいた。⁹
"I walked quietly."

Both 早い and 速い share the kana adverbial form はやく. The kanji distinguish "early in time" (早) from "fast in speed" (速) by 用字 (character-choice) convention rather than by grammar.⁷¹⁵¹⁶ The adverbial-formation rule (い → く) is identical for both.

Attributive vs. adverbial: position decides the form

The same adjective can modify a noun (attributive) or a verb (adverbial). The form follows the slot.³⁹ な-adjectives show this most cleanly: the surface difference is な before a noun and に before a verb, with both being cells of the same copula paradigm.¹¹⁰

静しずかな部屋へやで勉強べんきょうした。¹
"I studied in a quiet room."

静しずかに勉強べんきょうした。⁹
"I studied quietly."

For い-adjectives, the surface difference is い (attributive) vs く (adverbial), with no visible な anywhere. 大きい部屋 ("a big room") and 大きく書く ("write big") sit in the same paradigm.¹ The sibling article "Attributive vs. Predicative Adjectives in Japanese: Why 静か Needs な But 大きい Does Not" maps the full attributive picture.

Good to know

よく has two adverbial senses sharing one form

The adverb よく carries both "well / properly" (the direct adverbial reading of よい / いい) and "frequently / often."¹² Both senses are listed under the same etymology in lexicographic sources, since the "often" sense is a derived extension of the "well" sense within the same root.¹²

Context disambiguates in practice: よく寝る usually reads as "sleep well," while よく見る usually reads as "see often."¹² A learner who notices the ambiguity and asks whether this is one word or two is noticing something real. Historically it is one; in modern Japanese, it behaves like two.¹²

早く vs. 速く: same adverb, different kanji

The two い-adjectives 早い ("early in time") and 速い ("fast in speed") share the kana adverbial form はやく. The kanji draw the boundary between the senses in writing.⁷¹⁵¹⁶ By 用字 convention, 早 covers earliness and 速 covers speed, so 早く起きる reads "get up early" while 速く走る reads "run fast."⁷¹⁵¹⁶

The split is a writing-system distinction, not a grammar distinction. The adverbial-formation rule (い → く) operates the same way on both, and in kana-only writing the distinction collapses to はやく.⁷¹⁵¹⁶

早はやく起おきた。⁷
"I got up early."

速はやく走はしった。¹⁵
"I ran fast."

な / に / なる all descend from one Classical source

The modern attributive な and adverbial に are not arbitrary endings. They are the surviving cells of the Classical Japanese nari-adjective paradigm. In that older pattern, ni aru contracted to nari, whose attributive naru later contracted to な.¹⁰ The に cell survived from before the contraction.

The synchronic payoff is that the に in 静かになる, the に in 学生になる, and the な in 静かな部屋 are all reflexes of the same copula paradigm.¹⁰³ Three forms, one historical source.

連用形 is the same stem name on the verb side

The 連用形 ("continuative form") label that Japanese grammar textbooks attach to the verb stem (見 in 見ます, 食べ in 食べます) is the same name used for the く-stem of い-adjectives and the に cell of な-adjectives.¹ A reader who has met the verb stem will find that く and に are the adjective-side equivalents: the single cell from which the te-form, the negative, and the adverbial use all branch on the adjective side.¹²

The verb-side article "Verb Stem (連用形): The Most Productive Form in Japanese" covers the cell in full. For the adjective side, the takeaway is recognition rather than active production.

Class decides く vs に, never the surface sound

The single most common N5 error in this paradigm is using く with a な-adjective. Forms like 静かく and きれいく are ungrammatical. The correct adverbials are 静かに and きれいに.¹³ The trap is that きれい ends in い yet belongs to the な-class, with attributive きれいな部屋 and adverbial きれいに.¹¹³

静しずかに歩あるいた。¹³
"I walked quietly."

The diagnostic is the attributive form. Any adjective that needs な before a noun takes に before a verb; any adjective that takes bare い before a noun takes く before a verb. The final sound on its own is unreliable; the class membership is what decides.¹¹³

One stem, three suffixes on the い-class side

The く-form of an い-adjective is the 連用形 cell. The same cell feeds three downstream forms with three different following elements: bare く (adverbial use), くて (te-form), and くない (negative).¹² Treating these as three separate rules makes them harder to remember. Treating them as one stem with three suffixes turns three memorization tasks into one.²

The same is true on the な-class side, one step removed. The に cell feeds the adverbial use (静かに), and the parallel で cell feeds the te-form (静かで) and the negative chain (静かではない).¹¹⁰³ One paradigm, several cells, all linked by the copula on the な side.

The adverbial form does not host tense or politeness

The adverbial cell is a terminal shape: tense and politeness do not attach to it. Beginner-stage attempts like 早くです or 静かにでした are ungrammatical. Tense and politeness live on the verb that the adverb modifies.¹ "Ran quickly" is 早く走った (past tense on 走る, not on 早く), and "spoke softly" is 静かに話しました (politeness on 話す, not on 静かに).

The framing is structural rather than corpus-empirical: the 連用形 cell of an adjective is not itself a predicate. The tense / politeness slots that mark predicates therefore have nothing to attach to here.¹ The form is invariant.

See also

• Japanese Adjectives Overview: The Two Classes (い-形容詞 vs な-形容詞)
• い-Adjective Conjugation in Japanese: All Tenses and Forms
• な-Adjective Conjugation in Japanese: All Tenses and Forms
• Adjective Te-Form in Japanese: How to Link with くて and で
• Attributive vs. Predicative Adjectives in Japanese: Why 静か Needs な But 大きい Does Not
• The Japanese Verb Stem (連用形): The Masu-Stem and Its Uses

References

Footnotes

1. Wikipedia contributors. "Japanese adjectives." Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/wiki/Japanese_adjectives ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸ ↩¹⁹ ↩²⁰ ↩²¹ ↩²² ↩²³ ↩²⁴ ↩²⁵ ↩²⁶ ↩²⁷ ↩²⁸ ↩²⁹ ↩³⁰ ↩³¹ ↩³² ↩³³ ↩³⁴ ↩³⁵ ↩³⁶

2. Japanese with Anime. "~ku ～く Adverbial Copula." https://www.japanesewithanime.com/2022/04/ku-copula.html (limitation: language-learning blog used as secondary support; primary claim verified against ¹ and ¹⁷) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³

3. Japanese with Anime. "に Adverbial Copula." https://www.japanesewithanime.com/2019/10/ni-copula.html (limitation: language-learning blog; primary claim verified against ¹ and ¹⁰) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷ ↩¹⁸

4. Maggie Sensei. "Adjective + する & なる ( = suru & naru)." https://maggiesensei.com/2018/03/10/adjective-%E3%81%99%E3%82%8B-%E3%81%AA%E3%82%8B-suru-naru/ (limitation: pedagogy blog used for example-sentence inventory of the なる / する collocation; structural claims verified against ¹) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶

5. Hanabira. "JLPT Grammar Point: い-Adjective く + Verb." https://hanabira.org/japanese/grammarpoint/%E3%81%84-Adjective%20%E3%81%8F%20+%20Verb%20(i-Adjective%20+%20ku%20+%20Verb) (limitation: JLPT reference card; cited for JLPT-level pinning) ↩

6. Hanabira. "JLPT Grammar Point: な-Adjective + に + Verb." https://hanabira.org/japanese/grammarpoint/%E3%81%AA-Adjective%20%E3%81%AB%20+%20Verb (limitation: JLPT reference card; cited for JLPT-level pinning) ↩

7. Wiktionary. "早い" (hayai). https://en.wiktionary.org/wiki/%E6%97%A9%E3%81%84 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

8. Tofugu. "Japanese い-Adjective く Form as an Adverb." https://www.tofugu.com/japanese-grammar/i-adjective-ku-form-adverb/ (limitation: language-learning publisher used for example-sentence inventory and natural-register translations) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹ ↩¹² ↩¹³ ↩¹⁴ ↩¹⁵ ↩¹⁶ ↩¹⁷

9. Tofugu. "な-Adjectives: Japanese Noun-Like Adjective." https://www.tofugu.com/japanese-grammar/na-adjective/ (limitation: language-learning publisher used for example-sentence inventory) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

10. Wikipedia contributors. "Adjectival noun (Japanese)." Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/wiki/Adjectival_noun_(Japanese) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹ ↩¹⁰ ↩¹¹

11. Wiktionary. "良い" (yoi / ii). https://en.wiktionary.org/wiki/%E8%89%AF%E3%81%84 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

12. Wiktionary. "よく" (yoku). https://en.wiktionary.org/wiki/%E3%82%88%E3%81%8F ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

13. Hanabira. "JLPT Grammar Point: な-Adjective + く + Verb (the error class)." https://hanabira.org/japanese/grammarpoint/%E3%81%AA-Adjective%20%E3%81%8F%20Verb%20(na-Adjective%20+%20ku%20+%20Verb) (limitation: JLPT reference card; cited for documentation of the error class) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸ ↩⁹

14. Bunpro. "ではなくて・じゃなくて (JLPT N3)." https://bunpro.jp/grammar_points/%E3%81%A7%E3%81%AF%E3%81%AA%E3%81%8F%E3%81%A6-%E3%81%98%E3%82%83%E3%81%AA%E3%81%8F%E3%81%A6 (limitation: JLPT reference; cited for the ではなく / なくて register split) ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

15. Wiktionary. "速い" (hayai). https://en.wiktionary.org/wiki/%E9%80%9F%E3%81%84 ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

16. jpyokoso. "早はやい vs. 速はやい：Both Are HAYAI, But They Aren't The Same." https://jpyokoso.com/hayai-vs-hayai/ (limitation: language-learning blog used as secondary support; kanji distinction verified against ⁷ and ¹⁵) ↩ ↩² ↩³ ↩⁴ ↩⁵

17. Wikipedia contributors. "Japanese conjugation (continuative base)." Wikipedia, The Free Encyclopedia. https://en.wikipedia.org/wiki/Japanese_conjugation_(continuative_base) ↩