can 和must这些既可以表能力又可以表推测的情态动词在构成反义疑问句时是不同的，当他们表推测的时候，比如he can't be at home.这个句子的反义疑问句应该由can后面的动词决定，所以用isn't he? 当can 表示能力的时候，如，He can speak english. 这句是说他能讲英语，所以反义疑问句用can he? 以此类推。
这里几个句子都是表示 推测句子， 写反义疑问句的时候，改成不需要推测的句子。
He can't be at home. 他不可能在家， 改成他不在家， he isn't at home, is he.
he must be a doctor 改成he is a doctor , isn't he
You must have studied english for three years改成
you studied english for three yrear, didn't you
You have studied..., haven't you
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