表示否定推测只能用can't 和 couldn't

1.对现在情况的推测：must/can't be; must/can't be doing
He must be in the library.He can't be in the classroom.
(他一定是在图书馆.他不可能在教室里.）
She must be doing her homework.She can't be watching TV.
(她肯定是在做作业.她不可能在看电视.）
2.对过去情况或已经发生的情况的推测：must/can't have done
He must have left for Beijing.(他肯定已经前往北京了.)
(她不可能这么早就完成了工作.）