急に|きゅうに

1. 1Suddenly or unexpectedly; describes an action or change that happens abruptly.Ex. 1
2. 2At short notice; with little or no advance warning.

急に is a neutral, everyday adverb that describes sudden changes in weather, feelings, health, or plans. It often appears with verbs like 〜出す or 〜なる. Unlike いきなり, which stresses the lack of any warning, 急に focuses on the suddenness of the change itself.

• いきなりN2「いきなり」highlights acting with no prior notice or lead-up, with a strong impact and often a sense of surprise or displeasure; it can also mean skipping steps and doing something right off the bat, a sense that cannot be replaced with「急に」. When it modifies a noun, it becomes「いきなりの」.
• たとたんN3「たとたん」attaches to the ta-form of a verb and links two events, meaning "as soon as...," with the two happening almost simultaneously. 「急に」is an adverb that modifies a predicate to mean "suddenly," so the two cannot be interchanged.
• かと思ったら / かと思うとN2「急に」is an adverb that modifies a single predicate to express a sudden change, while「かと思ったら／かと思うと」connects two events, marking a contrasting second event that occurs right after the first; 「かと思ったら」can also mean "I thought... but...". The two cannot be interchanged.
• が早いかN1「急に」is an adverb meaning only that something happens suddenly. 「が早いか」attaches to the dictionary or ta-form of a verb and links two actions meaning "as soon as...," with the second action limited to things that have actually happened and a mostly written style; the two cannot be interchanged.
• なりN1「なり」attaches to the dictionary form of a verb and means the second event appears immediately after the first action, used in written narration of events that have happened. 「急に」is a neutral-register adverb meaning simply "suddenly" that needs no preceding action, so the two cannot be directly substituted.
• や否やN1「や否や」attaches to the dictionary form of a verb and links two actions meaning "as soon as...," with a strong written flavor. 「急に」is a neutral-register adverb that modifies a predicate to mean "suddenly" with no warning, so the two cannot be interchanged.

いきなり is a similar adverb but stresses that something happens without any warning or preparation, often with a more abrupt feel. たとたん, かと思ったら / かと思うと, が早いか, なり and や否や are clause-linking patterns that attach to a first action to say a second one followed immediately, whereas 急に simply modifies a single event.