1An adverb describing an action or change that happens swiftly or right away, close to 'quickly' or 'immediately' (e.g., standing up quickly).
彼は急に立ち上がった。
He stood up abruptly.
急に用事ができたので、先に帰ります。
Something urgent came up suddenly, so I'll head home first.
後ろから急に声をかけられて驚いた。
I was startled when someone suddenly called out to me from behind.
2An adverb describing something happening unexpectedly or without warning, close to 'suddenly' or 'out of the blue' (e.g., rain starting suddenly).
空が急に曇って、雨が降り出した。
The sky suddenly clouded over and it started to rain.
急に会議が中止になった。
The meeting was abruptly called off.
彼女は急に泣き出した。
She suddenly burst into tears.
3An adverb describing a slope, curve, or bend that changes sharply, used for steep roads or sudden turns.
この山道は急に下るので、運転には注意が必要だ。
This mountain road drops steeply, so you need to drive carefully.
カーブを過ぎると道が急に狭くなった。
After the curve, the road suddenly narrowed sharply.