13. 表示猜测的几种不同的方法及意义
　　一些情态动词与动词的完成式连用能够表示对过去情况的猜测或者未实现的可能性。

　　1) could have + 过去分词
　　A. 表示对过去可能发生的事情的推测，意为，“可能做了某事”。
　　He couldn’t have seen her yesterday.
　　They could have lost their way.
　　B. 表示某事在过去本有可能发生，但事实上并未发生，意为，“本可以做某事”。
　　We could have started a little earlier.
　　I could have killed her. It was a narrow escape.

　　2) may (might) have + 过去分词
　　A. 表示对过去情况的推测，意为，“可能已做某事”。
　　He may have heard the news.
　　I might have come to a wrong conclusion.
　　B. 表示一种未实现的可能性，即本可以做某事，而实际上并没有做，有时含有抱怨的口吻。
　　It was a narrow escape. You might have killed yourself.
　　A lot of men died who might have been saved.

　　3) must have + 过去分词
　　表示逻辑上的必然性，即按照某些现象推断过去肯定发生过的某事。
　　She must have made a big mistake.
　　The city at one time must have been prosperous, for it enjoyed a high level of civilization.

　　4) needn’t have + 过去分词
　　表示过去不必做某事，但已经做了，即本可不必做某事。
　　You needn’t have woken me up. I don’t have to go to work today.
　　He needn’t have watered the flowers, for it is going to rain.

　　5) should / ought to have + 过去分词
　　表示过去应该做某事而实际上没有做。
　　You should / ought to have done what your parents told you. (But you failed to do it.)
　　You should / ought to have been more careful. (But you weren’t.)

　　6) shouldn’t / oughtn’t to have + 过去分词
　　表示过去不应该做某事而实际上却做了。
　　You shouldn’t / oughtn’t to have crossed the road when the lights were red. (But you did.)
　　They shouldn’t / oughtn’t to have left so soon. (But they did.)

　　7) would have + 过去分词
　　表示对现在或将来某时之前业已完成的动作的推测。可译为，“可能”、“也许”、“想必”。
　　He would have arrived by now.
　　She would have recovered by then.