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“（昨天）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“（昨天）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，但实际上作者还是没有规划好，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：
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did’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. 我无法想象他竟然这样做。
三．情态动词被动关系的主动表达法
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.