急に【きゅうに】

急に means "suddenly": something happens or changes without warning. It is the adverb form of the な-adjective きゅう急, and it goes before the verb, often with a verb of change like なる or one ending in だす出す ("start to").

急に or 急いで

急に is about when something happens: without warning. いそいで急いで is about how fast you act: in a hurry. They come from the same kanji, and English "quickly" pulls learners toward 急に.

• × 遅れるから、急に行こう → 遅れるから、急いで行こう

急に or 突然

とつぜん突然 also means "suddenly", and in most sentences the two swap. 突然 stresses that nobody saw it coming and is more common in writing. 急に is the everyday word, especially for a change in how something is.

Usage notes

These go beyond "suddenly" before a verb.

急には with a negative

With は and a negative, 急に means "at such short notice".

急な before a noun

Before a noun, the adjective form is きゅう急な: きゅう急なようじ用事, "something urgent that came up".