急に and 突然: rapid changes and unexpected events
Compare rapid changes with events occurring without warning, while recognizing overlap between 急に and 突然.
A state changes quickly
夕方から急に寒くなりました。 says that it suddenly became cold from the evening onward. 急に, read きゅうに, can emphasize a change over a short time.
急に予定を変えて、すみません。 apologizes for a sudden change of plans. It concerns changing them without much notice and does not specify the reason.
An event without warning
突然、電気が消えました。 says that the lights suddenly went out. 突然, read とつぜん, highlights an event occurring unexpectedly.
突然の連絡で驚きました。 says that unexpected contact surprised the speaker. Before a noun, use a form such as 突然の rather than mechanically constructing 突然な連絡.
Overlapping contexts and forms
Both 雨が急に降り始めました and 雨が突然降り始めました can describe rain starting suddenly. Their meanings can overlap in a real situation.
急な変更 means a sudden change; 急に変える means changing something suddenly. 急な modifies a noun and 急に an action, so choose the form for its position.
Can 突然の連絡 and 急な連絡 be freely changed to 突然な and 急の?
Show the answer and why
No. Use 突然の and 急な in these expressions. Similar meanings do not guarantee identical forms before nouns.