i saw Ann in the library yesterday.-you()her.she is still ab

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i saw Ann in the library yesterday.-you()her.she is still abroad
为什么括号中选的是can't have seen?看到ANN不是过去发生的吗?为什么还用现在完成时呢?求讲解

又看见你的提问了,感觉有些亲切
这里有个知识点：情态动词加现在完成时,表示对过去的推测
must have done 表示昨天肯定怎样
can have done 表示一定怎么样,上述两种都是肯定性推测
should have done ,有虚拟语气,本来应该怎么样,结果没怎么样,有责备口吻
再问：我也是；那could呢？选项里有could not see，为什么不能选？他也表示虚拟语气吗？
再答：表推测的语气的强弱在这里，要表达一种很肯定的推测，could 的语气没有can 的语气强
再问：那么mustn't have seen也是可以的？表示昨天你肯定没有看到他？
再答：不能，mustn't 只能表禁止，不能表推测

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