can / could be doing sth,can't / couldn't be doing sth是表示现在进行时吗? can't和couldn't怎么选呢?
can't/ couldn't be doing sth 都是情态动词的推测性用法,表示说话人推测某件事不可能或者不太可能正在发生。如果说话人推测时信息很足,认为绝对不可能某事正在发生,就用can't be doing. 如果自己推测的准确性只有60%左右,则用couldn't be doing.
Tom can't be sleeping now. I saw him in the supermarket five minutes ago. 不可能
Tom couldn't be sleeping now. He has lots of homework to do. 不大可能
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