表推测时must,can怎么用

当表示推测用法时：must的意思是：必定,肯定,表示把握最大,可能性最强,通常用于肯定句,比如：the man must be our math teacher,那人一定是我们的数学老师.can 表示推测时意思是：可能,也许,常常用于疑问句或者否定句,用于疑问句时表示说话人的怀疑,不确定；can he come here so early tomorrow?意思是明天他能来那么的早吗?用于否定句时意思是：“不可能” ,Mr Smith can't be at home because he has gone to America .,需要说明的是,当含有must的表示推测判断的句子在变成否定句时应该用can't,比如：
they must have finished their homework,否定句是：
they can't have finished their homework.
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