～く
adjective as adverb
The ～く form (連用形) turns an i-adjective into an adverb, so it modifies a verb instead of a noun, much like English “-ly”: 早く起きる (はやくおきる) = get up early.
1. Turning an adjective into an adverb
Swap the final い for く and place the word before the verb it describes.
毎朝、早く起きる。
“I get up early every morning.”
- 毎朝（まいあさ）= every morning
- 早く（はやく）= early (adverb)
- 起きる（おきる）= to get up
2. ～くなる: “become ...”
With the verb なる (“to become”), the ～く form describes a change of state.
最近、寒くなった。
“It has gotten cold recently.”
- 最近（さいきん）= recently
- 寒く（さむく）= cold (adverbial)
- なった = became
How to form it
Drop the final い, add く. This works for every i-adjective; the only quirk is that いい (“good”) uses its older form よい, giving よく.
- 早い（はやい）= early → 早く（はやく）= early (adverb)
- 大きい（おおきい）= big → 大きく（おおきく）= greatly / (become) big
- よい／いい = good → よく = well / often
Key idea
～く is the “-ly” of Japanese: it points the adjective at the verb instead of a noun, describing how something is done or what something turns into when used with なる.
Other N5 grammar
Updated .