急に【きゅうに】

急に (きゅうに) is an adverb meaning "suddenly", "abruptly", or "all of a sudden". It describes an event or change that occurs unexpectedly and without warning. It is formed from the な-adjective きゅう急 (きゅう, "sudden/急な") plus the adverbial particle に.

急に typically modifies verbs and is placed before the verb or verb phrase it describes.

It is used with verbs that describe changes of state, weather, emotions, or unexpected actions.

急に vs. 突然

急に and とつぜん突然（とつぜん）are very similar in meaning, both translating to "suddenly." However, 急に tends to emphasize the speed or abruptness of the change, while 突然 emphasizes that the event was completely unexpected. In many cases they are interchangeable, but 急に is somewhat more common in everyday conversation.

Negative usage

急に can also be used with negative forms to express that something cannot be done quickly or on short notice.

Here, は is inserted after 急に for contrast, emphasizing that doing it suddenly is the problem.