couldn't have done
1.对过去情况推测,表示：不可能做了某事
比如：He couldn't have come here yesterday.他昨天不可能来了这里.
2.虚拟,意思还是,不可能做...
比如 ：If you hadn't study hard,you couldn't have passed the exam
如果你没努力学习,你就不可能通过这次考试.
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