He must have been asleep at that time,(wasn't) he?为什么是用 wasn

情态动词表示推测的句子,完成反义疑问句时,首先要去除推测,还原成普通句子。故He must have been asleep at that time根据at that time 为过去时间,需还原成：He was asleep at that time。中间为be动词的肯定式,故为：wasn't he。 再问： 那“Tom must have seen the movie last night，didn't he ？”这里为什么用“didn't”？ 再答： last night该用一般过去时，故原句还原成： Tom saw the movie last night, didn't he? 但：Tom must have seen the film before, hasn't he? 原因是：before 置于句末常用完成时，故夲句应还原成：Tom has seen the film before,hasn' t he?