Sudden means happening quickly and unexpectedly.
He had been deeply affected by the sudden death of his father-in-law.
岳父的突然辞世对他影响很大。
It was all very sudden.
一切都非常突然。
all of a sudden
If something happens all of a sudden, it happens quickly and unexpectedly.
All of a sudden she didn't look sleepy any more.
突然间,她看上去不再困倦了。