Question

一道英语选择题请详细解释

-----We did not see him at the lecture yesterday.
------You______him,since I went to an international conference with him,
B.can't have seen C.couldn't see
can't have done/shouldn't have done/mustn't have done分别什么意思？

Answer 1

can't have done/shouldn't have done都是对过去的虚拟,不能用mustn't have done表示对过去的猜测
can't have done是过去不可能做
shouldn't have done是本不应该做，但是已经做了
选B是因为这是对过去的猜测。第二个人说他跟第三个人一起参加国际会议了，但是他并不能完全肯定地说第三个人先回来了。他说第三个人不可能在会议上，只是对过去的比较肯定的猜测

Answer 2

A 是虚拟语气
情态动词＋ have done都是表示对过去动作的推测
must have done 过去一定做了某事
should /need have done 本来应该／需要做某事（却没做）
should't/needn't have done本来不应该／不需要做某事（却做了）
may/might have done 过去可能做过某事
could have done过去有能力做某事

Answer 3

正确答案是C。

can't/couldn't 不可能

本题真正的干扰项应该是couldn't have seen（基于过去时间的“不可能已经”，用来否定对方使用一般过去时态的描述或猜测）。

其它几个选项的语意只需在原有情态动词语意前面加上一个“本”字即可。

Answer 4

B.can't have seen