注意情态动词完成式的用法有两方面的含义：
1.表示已经发生的情况
(1)must have +过去分词，表示对已发生情况的肯定推测，译为“(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)cant/couldn’t have +过去分词，表示对已发生情况的否定推测，译为“(昨天)一定没……”。 如：
Mary couldn’t have received my letter; otherwise she would have replied before now.
(3)may/might have +过去分词，表示对已发生的事情做不肯定、可能性很小的推测，或事实上根本没发生，译为“也许……”。如：
At Florida Power’s Crystal River plant, a potentially serious leakage of radioactive water may have been unknowingly caused by an electrician.
2.表示虚拟语气
(1)needn’t have+过去分词，表示做了不必做的事，译为“其实没必要……”。如：
As it turned out to be a small house party, we needn’t have dressed up so formally.
(2)should /should not have +过去分词，表示应该做某事但实际上未做，或本不应该做但实际上做了, 译为“本(不)应该……”。如：
I regret having left the work unfinished; I should have planned everything ahead carefully.
(3)ought to have+过去分词，表示动作按理该发生了，但实际上未发生，译为“该……”，与should的完成式含义类似。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+过去分词，表示过去本来可以做但却未做，译为“完全可以……”。如：
What you said is right, but you could have phrased it more tactfully.
(5)may/might have + 过去分词，表示过去可以做但实际未做，译为“(那样)也许会……”。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.几个情态动词常考的句型
(1)may/might (just) as well“不妨，最好”，与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，怎么也不过分”。注意这个句型的变体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为used to (do)的否定式。
(4)should 除了“应该”一层意思外，考研大纲还规定要掌握其“竟然”的意思。如：
I didn’t expect that he should have behaved like that.
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