2009年12月大学英语四六级考试语法精要(五)

　　12. It is (about/high) time (that) 句型

　　该句型表示“(早)该做…”，其后的定语从句中的谓语动词要用虚拟语气表示。在测试中从句的谓语动词用一般过去时。

　　It is time that we went to bed.

　　It is high time that somebody taught you to behave yourself.

　　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.