4.对已发生的事情进行推测：
句式：must/may/might/can/could＋have done
例如：There’s no lignt in the room. They must have gone to bed./She knows nothing about the film. She can’t/couldn’t have seen it./Can/Could he have been a doctor?/He may/might(not)have been a teacher./They must have been watching TV at nine last night./Can/Could she have stayed in Beijing last year?
