can not have + p.p. 指现在对过去的动作认为不可能

She can not have gone to the movies with Danny at eight last night.

她昨晚八点的时候不可能跟Danny去看电影。(那个时候她跟我在家一起看电视。We were watching TV at home at that time.)

You can't have seen him this morning. He left for America yesterday.

你今天早上不可能看到他，他昨天就去美国了。

could 可表示礼貌的请求或建议，并非指过去

Could you please give me your phone number?

可以给我你的电话号码吗？

Could you wait for me ten more minutes? 你可以再等我十分钟吗？

Could I stay here a little longer? 我可以在这儿多待一会儿吗？