一。情态动词的现在完成式的用法
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/cann‘t;need/needn‘t;may/mayn‘t;might/mightn‘t;should/shouldn‘t;
ougtht等情态动词+完成式表示的意思是有一定区别的
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“()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)
2)can‘t / couldn‘t have+过去分词，表示对已发生情况的否定推测，译为“()一定没……”。如：
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)
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，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
(没有必要穿的那么正式，体现是说话者的建议，实际结果是否真的穿的很正式没有确定，答案为D)
2)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“本不应该……”。如：
I regret having left the work unfinished; I should have planned everything ahead carefully.
我本来应该事先认真地把每件事情规划的很好，但实际上还是没有规划好，以至工作没有完成。
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……”。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.
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.
二。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;
Since the flight was cancelled， you might as well go by train.
既然航班已经取消了，你不妨乘火车吧。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：liuxuepaper.com