く / に

In Japanese, adjectives can be turned into adverbs (words that modify verbs, similar to "-ly" in English) by changing their ending.

い-adjectives → く

Drop the final い and add く.

• はやい早い (early/fast) → はやく早く (early/quickly)
• おおきい大きい (big) → おおきく大きく (largely/in a big way)
• やさしい優しい (gentle) → やさしく優しく (gently)

な-adjectives → に

Drop な and add に.

• しずか静かな (quiet) → しずかに静かに (quietly)
• じょうず上手な (skillful) → じょうず上手に (skillfully)
• きれいな (beautiful/clean) → きれいに (beautifully/cleanly)

How adverbs work in sentences

Adverbs typically come before the verb they modify. They describe how an action is performed.

いい → よく

The adjective いい (good) is irregular. Its adverb form is よく (well), not ×いく.

Adverbs from adjectives vs. く form for other uses

The く form of い-adjectives is also used to connect to other words and in negative constructions (e.g., おおきくない大きくない "not big", やすくて安くておいしい "cheap and delicious"). As an adverb, it specifically modifies a verb to describe the manner of an action.