注意情態動詞完成式的用法有兩方面的含義：

　　1.表示已經發生的情況

　　(1)must have +過去分詞，表示對已發生情況的肯定推測，譯為“(昨天)一定……”。如：

　　My pain must have been apparent the moment I walked into the room, for the first man I met asked sympathetically: “Are you feeling all right?”

　　(2)can�餿/couldn’t have +過去分詞，表示對已發生情況的否定推測，譯為“(昨天)一定沒……”。 如：

　　Mary couldn’t have received my letter; otherwise she would have replied before now.

　　(3)may/might have +過去分詞，表示對已發生的事情做不肯定、可能性很小的推測，或事實上根本沒發生，譯為“也許……”。如：

　　At Florida Power’s Crystal River plant, a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.

　　2.表示虛擬語氣

　　(1)needn’t have+過去分詞，表示做了不必做的事，譯為“其實沒必要……”。如：

　　As it turned out to be a small house party, we needn’t have dressed up so formally.

　　(2)should /should not have +過去分詞，表示應該做某事但實際上未做，或本不應該做但實際上做了, 譯為“本(不)應該……”。如：

　　I regret having left the work unfinished; I should have planned everything ahead carefully.

　　(3)ought to have+過去分詞，表示動作按理該發生了，但實際上未發生，譯為“該……”，與should的完成式含義類似。如：

　　The porter ought to have called the fire-brigade as soon as he saw the fire in the stock, which went up in smoke.

　　(4)could have+過去分詞，表示過去本來可以做但卻未做，譯為“完全可以……”。如：

　　What you said is right, but you could have phrased it more tactfully.

　　(5)may/might have + 過去分詞，表示過去可以做但實際未做，譯為“(那樣)也許會……”。如：

　　It might have been better to include more punchy statistics and photos of equipment in the introduction to further assist first-time office automation managers.

　　3.幾個情態動詞常考的句型

　　(1)may/might (just) as well“不妨，最好”，與had better相近。如：

　　Since the flight was cancelled, you might as well go by train.

　　(2)cannot/can’t…too… “越……越好，怎么也不過分”。注意這個句型的變體cannot…over…。如：

　　You cannot be too careful when you drive a car.

　　The final chapter covers organizational change and development. This subject cannot be overemphasized.

　　(3)usedn’t或didn’t use to為used to (do)的否定式。

　　(4)should 除了“應該”一層意思外，考研大綱還規定要掌握其“竟然”的意思。如：

　　I didn’t expect that he should have behaved like that.