a． may /might /have done1．表示对过去事实不大肯定的推测，常译为“可能已经”。
they might have taken a later train. 也许他们乘坐的是晚一点儿的火车。he seems to know the city quite well. he may have been there before. 他似乎对那个城市很熟悉，他以前可能去过那儿。
2．表示“本来应该或可以做但没有做某事”，含有轻微的责备口吻或遗憾。you might have given him more help, though you were busy. 你本来应该给他更多的帮助，虽然你很忙。the proposal might have been refused. 这个建议本该拒绝的。b． cancould have done1．表示对过去事实的推测，常用于否定句和疑问句。he can't couldn't have watched tv yesterday for he knew they would have an exam. 昨天他不可能看电视，因为他知道快要考试了。i don't know why she didn't call me yesterday. can (could) she have forgotten my telephone number 我不知道昨天她为什么没给我打电话，难道她会把我的电话号码忘了吗？he didn't come to school yesterday. can he have been ill 他昨天没来学校，难道他生病了？2．could可表示对过去能做而未做的事感到惋惜、遗憾，可译成“本来是可以……的”。why didn't sophie apply for the job she could have got it. 为什么索菲不申请这项工作？她本来是能得到的。why did you stay at a hotel when you went to new york you could have stayed with barbara.你到纽约时为什么住宾馆？你完全可以和芭芭拉住在一起的。given more time, we could have done the work better. 如果给我们更多的时间，我们完全可以把这项工作干得更好。
