12. It is (about/high) time (that) 句型

　　該句型表示“(早)該做…”，其后的定語從句中的謂語動詞要用虛擬語氣表示。在測試中從句的謂語動詞用一般過去時。

　　It is time that we went to bed.

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

　　13. 表示猜測的幾種不同的方法及意義

　　一些情態動詞與動詞的完成式連用能夠表示對過去情況的猜測或者未實現的可能性。

　　1) could have + 過去分詞

　　A. 表示對過去可能發生的事情的推測，意為，“可能做了某事”。

　　He couldn’t have seen her yesterday.

　　They could have lost their way.

　　B. 表示某事在過去本有可能發生，但事實上并未發生，意為，“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 + 過去分詞

　　A. 表示對過去情況的推測，意為，“可能已做某事”。

　　He may have heard the news.

　　I might have come to a wrong conclusion.

　　B. 表示一種未實現的可能性，即本可以做某事，而實際上并沒有做，有時含有抱怨的口吻。

　　It was a narrow escape. You might have killed yourself.

　　A lot of men died who might have been saved.

　　3) must have + 過去分詞

　　表示邏輯上的必然性，即按照某些現象推斷過去肯定發生過的某事。

　　She must have made a big mistake.

　　The city at one time must have been prosperous, for it enjoyed a high level of civilization.