情态动词现在完成式主要有两个功能：表示已经发生的情况和表示虚拟语气。在这两个方面 must/mustn't; can/can' t; need/needn't; may/may not; might/mightn't; should/shouldn't; ought 等情态动词+完成式表示的意思是有一定区别的。
a) must have +过去分词，表示对已发生情况的肯定推测，译为“一定…” 如：
My pain______ apparent the moment I walked into the room, for the first man I met asked sympathetically:" Are you feeling all right?"
[A] must be [B] had been [C] must have been [D] had to be （答案为C）
Mary______ my letter; otherwise she would have replied before now.
[A] couldn't have received [B] ought to have received
[C] has received [D] shouldn't have received (答案为A)
c) 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，"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.
a) needn't have + 过去分词，表示做了不必做的事, 相当于" didn't need to do"译为“其实没必要……”如：
You needn't have come over yourself. As it turned out to be a small house party, we ______so formally.
[A] needn't dress up [B]did not need have dressed up
[C] did not need dress up [D] needn't have dressed up (答案D)
b)should have +过去分词，表示应该做某 事但实际上未做，译为“本应该…” should not + have 过去分词，表示本不应该做某事但实际上做了，译为译为“本不应该…”如：
I regret having left the work unfinished; I should have planned everything ahead received carefully.
c) 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.
d) 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…”, 这点与 ought/should/ have +过去分词用法相似。如：
What you said is right, but you could have phrased it more tactfully.
e) 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.
1) may/might (just) as well “不妨， 最好” ，与 had better 相近；
Since the flight was cancelled, you might as well go by train. 相当于 you had better 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. 驾车时候，越小心越好。
The final chapter covers organizational change and development. This subject cannot be over emphasized.
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.
1) want, require, worth(形容词) 后面接doing也可以表示被动意义。
Your hair wants cutting.
The book is worth reading.
The floor requires washing.
2) need 既可以用 need to be done 也可以使用 need doing，两种形式都表达被动的意义
The house needs painting= the house needs to be painted.
The watch needed repairing= the watch needed to be repaired.