大学英语四级语法精要：情态动词（二）
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can’t, couldn’t表示否定的推测:
She can’t be serious.
A more suitable book can’t be found. (It is not possible to find a more suitable book.)
He couldn’t (can’t) be over fifty.
She can’t be serious.
A more suitable book can’t be found. (It is not possible to find a more suitable book.)
He couldn’t (can’t) be over fifty.