急に|きゅうにkyuuni

1. adverbswiftly; rapidly; quickly; immediately; hastily; hurriedly
   気苦労きぐろうで彼かれは急きゅうにふけた。↗

   Care aged him quickly.
2. adverbsuddenly; abruptly; unexpectedly
   彼かれは急きゅうに居直いなおった。↗

   He changed his attitude suddenly.
3. adverbsharply (of a slope, bend, etc.); steeply

• SynonymにわかにWrittenRefers to something happening abruptly and unexpectedly, as in 「にわかに曇る」. Close to 「急に」 for sudden change, but it leans toward written prose.
• SynonymたちまちNeutralMeans reaching a state in an instant, as in 「たちまち売り切れる」, stressing how little time it takes; 「急に」 puts the weight on the suddenness of the change.
• SynonymふとNeutralCovers things that happen unintentionally or by chance, as in 「ふと思い出す」; 「急に」 has no nuance of being unintentional or accidental.
• SynonymさっとNeutralCovers quick actions and the sudden arrival of wind or rain; 「急に」 stresses that a change is sudden and does not necessarily suggest nimbleness.
• SynonymパッとCasualColloquial; captures an instant action, change or spread, as in 「パッと明るくなる」; 「急に」 states only suddenness, not the manner, and has no eye-catching sense.
• Synonym急激きゅうげきNeutralA na-adjective for changes or movement that are both fast and violent, used adverbially as 「急激に」; 「急に」 does not necessarily stress the intensity.
• Synonym急きゅうNeutralA na-adjective that modifies nouns as 「急な」, for things like urgent business or steep slopes; 「急に」 is an adverb modifying actions and changes.
• SynonymいきなりCasualColloquial; for things happening without warning or preparation, as in 「いきなり殴る」; 「急に」 can also mean a fast change, without stressing an interruption.
• Synonym突然とつぜんNeutralFor events with no warning; it modifies nouns as 「突然の」 and narration with it is fairly objective; 「急に」 can additionally stress how fast a change is.
• Synonym突如とつじょWrittenMostly written; for sudden events without warning, common in news reports and narration. Close to 「急に」 in the sudden sense, but 「突然」 serves in daily speech.
• Synonym不意にふいにNeutralStresses that something catches you off guard, as in 「不意に声をかけられる」; 「急に」 is broader and can simply stress the speed of a change.
• SynonymふいとNeutralDescribes acting suddenly and unexpectedly, painting the manner of the move. Everyday speech would use 「急に」, which does not picture how the action looks.
• SynonymふっとNeutralSuggests a sudden puff — it can picture blowing air or a gust of wind; 「急に」 merely states that something is sudden and swift, without evoking that sound.
• SynonymつとWrittenMostly written; can describe acting suddenly but also means staying perfectly still. For sudden occurrences it swaps with 「急に」, but not for the motionless sense.
• SynonymついとWrittenDepicts a sudden, swift action and is mostly written. Everyday speech uses 「急に」, which does not highlight how brisk the action is.
• SynonymひょいとCasualColloquial; describes acting offhandedly, with a light, by-chance feel; 「急に」 carries no sense of lightness or coincidence.
• SynonymばっとCasualColloquial; paints a sudden, swift, forceful action or unfolding; 「急に」 only says something is sudden and does not stress the vigour of the action.
• SynonymはたとWrittenMostly written; used for suddenly noticing or stopping dead, and for the sound of a collision, as in 「はたと気づく」; 「急に」 evokes no sound or impact.
• SynonymつっとWrittenRare and mostly written; pictures a quick, smooth movement. In daily speech you would say 「さっと」; 「急に」 puts more weight on something happening suddenly.
• Related俄然がぜんWrittenMostly written; marks an abrupt turn and can also convey 'absolutely' or 'by far'; 「急に」 is close in the sudden sense but has no such intensifying use.
• RelatedやにわにWrittenMostly written; covers acting without warning or at once. In everyday speech 「急に」 or 「いきなり」 is used, without this descriptive tone.
• Related忽然こつぜんWrittenMostly written, usually as 「忽然と」, and typically describes things suddenly appearing or vanishing; 「急に」 is more everyday and applies to other changes too.
• RelatedとみにWrittenMostly written; describes a sudden, rapid change, as in 「とみに衰える」. Close to 「急に」 for sudden change, but it can also mean frequently or in great measure.
• Related突としてとつとしてWrittenRare and mostly written; means happening suddenly with no warning. Everyday speech prefers 「突然」; 「急に」 can also stress a swift change.
• Antonym徐々にじょじょにNeutralAntonym: describes gradual, bit-by-bit change, as in 「徐々に慣れる」; its slow change stands opposite the abrupt change of 「急に」.
• AntonymだんだんNeutralAntonym: 「段々」 describes step-by-step change, as in 「段々暖かくなる」, close to 「徐々に」; its gradual change is the opposite of the sudden change of 「急に」.

Use 「急に」 when a situation changes suddenly. Choose 「いきなり」 for things that happen without warning, 「突然」 for objective narration, 「急激」 for drastic change, 「ふと」 for unintentional recollections, and 「たちまち」 for reaching a state in an instant. The antonyms are 「徐々に」 and 「段々」.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

1. 突然とつぜんに。にわかに。頓とみに。