题目内容：

be fast asleep与fall asleep的区别.
顺便分别造句来说明他们的不同.

优质解答

He's fast asleep 他睡得很熟,动作可延续
He fell asleep an hour ago.
fall asleep 入睡,是短暂动词,不能跟一段时间连用
如：He has been fast asleep for eight hours.
就不能用fall