情态动词
情态动词一般用法，大家都比较熟悉，但是，到了高中阶段，它的用法外延的比较多，很难理解和掌握。这里，就其考点列举如下：
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; oughtn’t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
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
A couldn’t have received B ought to have received
C has received D shouldn’t have received
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：
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
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：
You ought to have told him that the paint on that seat was wet..
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 done it better.
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：
You may have noticed something while you were doing this task.
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,意为“最好，还是，不妨”。
You may as well repeat the experiment.
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。
3) usedn’t 或did’t use to 为used to (do) 的否定式。
Tom used not to rise at six every morning.
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。
二． 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 work 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.
补充：
had better, should, ought to是重要的情态动词，也是高考重要考点，学习时应注意下列几点：
一、了解相互间关系
had better(最好), should(应该)与ought to（应该）均为提建议、阐述观点的情态动词，其后接动词原形。should 比had better语气强，ought to语气最强。一般情况下should与ought to可通用。
例1：You had better go there at once. 你最好立即去那里。
例2：You ought to / should work hard.你应该努力工作。
值得注意的是：should还有"竟然"之意，表出乎意料。
例：You can't imagine that a well-behaved gentleman should be so rude to a lady. 你真是难以想象这么一个有风度的先生对一个女子是如此之粗鲁。
二、 掌握句型变换方法
had better, should, ought to作为情态动词，各种句型变换均在自身形式上作变化。
（一）否定句
had better, should, ought to用于否定句时，否定词均位于其后，具体形式为：had better not do, shouldn't / should not do, oughtn't to do。
例1：You had better not start at this time. 此时你最好别出发。
例2：He shouldn't be careless in class. 他上课时不应粗心。
例3：She oughtn't to waste time. 她不应该浪费时间。
（二）一般疑问句
had better, should, ought to用于一般疑问句时，分别将had, should, ought提至句首。
例1：Had he better start early at once? 他最好立即出发吗？
例2：Should you do like this? 你应该这么做吗？
例3：Ought he to go there? 他应该去那里吗？
（三）反意疑问句
陈述句谓语动词含had better, should, ought to时，反意疑问句为：had, should与ought+主格人称代词或hadn't, shouldn't, oughtn't+主格人称代词。
例1：You had better not go home, had you? 你最好别回家，是吗？
例2：You should study hard,
shouldn't you? 你应该认真学习，是吗？
例3：He oughtn't to speak in class, ought he? 你上课不应该讲话，是吗？
三、 掌握其完成式用法
had better没有完成式，should与ought to完成式意义与用法完全相同。
（一）should/ought to have done 意为本来应做但实际没有做，表达遗憾的感情色彩。
例：There was a lot of fun at yesterday's party. You ________ come, but why didn't you?
A. must have B. should C. need have D. ought to have
析：答案D。 该空含你本来应来但实际未来之意，应填ought to have。
（二）should / oughtn't to have done，意为本来不应做但实际已做，表达遗憾的感情色彩。
例：I was really anxious about you. You ________ home without a word.
A. mustn't leave
B. shouldn't have left
C. couldn't have left
D. needn't leave
析：答案B。 该空含你本不应该一句话不说就离开家但实际已这么做之意，所以应填shouldn't have left。
责任编辑：李芳芳