爱华网情态动词表示说话人的语气和情态。如需要、可能、愿意、怀疑等。情态动词有一定的词义，但不完整，必须和主要动词的原型一起构成谓语。常用的情态动词有：can, could, may, might, must, have to, ought to, need, dare, shall, will, should, would, be able to,及半情态动词be to do, had better等。
情态动词的基本用法：
（一）can“能够”
1.表示体力或脑力方面的能力，或根据客观规律能做某事的能力。
(1)can表“能够”时，既可指现在也可指将来。
(2)can与be able to的区别：
① 表“能力”时二者同义
No one can / is able to do it.
② be able to常侧重经过努力有能力做到，有多种时态。can侧重自身的能力，只有一般现在时和一般过去时。
③ was / were able to do表示通过努力克服困难或在一定的条件下成功地做了某事，侧重结果，相当于managed to do或succeeded in doing...。could仅表自身的能力或本领，但未必成功做到。
With the help of the firemen, they were able to leave the burning house. He was able to swim across the river.
In the end, we were able to rescue the workers trapped in the fire.
2.表示“许可，允许”。在疑问句中表示请求，许可；否定句中表示“不许”。此时常与may通用：
May/Can I do it?
This sort of thing can't go on! 这类事情不能再继续了！ 注意 can not有时可以表示“禁止”相当于mustn't：
You can't（mustn't，are not allowed to）play football in the street.你不可以在大街上踢球。
3.表示推测，强调客观可能性：“可能、会”。主要用于疑问句和否定句。表示推测、估计时，后边可根据不同情况用动词原形、进行式、完成式。
She can't be waiting for you. He can't have gone there alone.
4.表惊讶、疑虑。
How can it be that he was late for the opening ceremony?
（二）could
1．表“能力”“可能性”“许可”，为can的过去式。 He said he could go.
2. could用于一般现在时中表委婉、客气地提出问题或陈述看法。（此时不是can的过去式，要用can回答以显得爽快。此时can和could没有时间的差别，只是could语气更客气些。） -Could I borrow your pen? - Yes, you can.
（三）may, might (may的过去式为might）
1.表示许可，有“可以”的意思，询问或说明一件事可不可以做，有时可以用can互换。 You may go now.
否定式may not表示“不可以、禁止、阻止”等，常用must not（＝mustn't）代替。may用于疑问句中时，回答多避免用may，以免显得太严肃或不客气。
2.表推测：“或许、可能”。表示一件事或许会发生，只用在陈述句中。 It may rain tomorrow.
3. may可以表祝愿，一般放在句首构成倒装句式：
May you succeed! =I hope you may succeed.祝您成功! May that day come soon! 希望这一天早日来到！
4. might是may的过去式，表可以做的事或可能发生的事。 She suggested a few books which I might buy.
5. might用于一般现在时比may委婉，或表示现实的可能性更小一些。 He might come soon.
6. may (just) as well, might as well.“不妨…”（可以用来提出建议）。它们比had better显得谦逊或不那么有把握，所以had better通常是不用于长者或上司的。同时，had better有时带有威胁口吻。（may well为“完全能；满可以；很可能”，表示有可能或有充分理由做某事。well是may的程度副词）
I needn't wait; I may as well tell you now. 我不必等，我不妨现在就告诉你。
Since it's a fine day, we might as well walk. 既然是好天气，我们一不妨走走。
It may well be that the question is still under discussion. 那很可能是该问题仍在讨论中。
（四）must
1.表示必须要做的事，意为“必须”，否定式mustn't表示不准、不许、禁止等。在回答带有must的问句时，不用mustn't，而用needn't或don't have to。 I must go now.
Must I go home? No，you needn't./ Yes, you must.
must在表“必须”时与have to相近，have to强调客观需要，must着重主观看法。另外must不像have to能有更多的时态。
2.表推测，暗含很大的可能性，意为“一定、肯定”，只用于“肯定句”中。此时其否定式为can't,“绝非”。
He must be over sixty. There must be a mistake.
3.偏偏、硬要、干嘛（指做令人不快的事） Must you shout so loudly? 你干嘛这么大声喊？
（五）shall
1.表示命令、履行义务、意愿、许诺、警告、强制、威胁、决心等，一般用于二、三人称中，并用于陈述句。
You shall have my answer tomorrow. （允诺） You shall be sorry for it one day, I tell you.（警告）
You shall do as I tell you. （命令或吩咐） We shall go, whatever may happen.（决心）
2.征求听话人意见和指示，用在一、三人称的疑问句中。Shall he come to see you? ＝Do you want him to see you?
（六）should should是shall的过去式，但它同时又是一个有单独意义的情态动词。
1.表达职责、义务，作“应当”讲，常与ought to互换。但ought to侧重反映客观情况，should侧重表示主观看法。
2.表示预测可能性，并译作“可能，（按道理）应该”。 They should come here now.
3.用于委婉、谦逊地提出意见或建议。 I should think you are right.我以为你是对的。
4. should用于某些从句中表虚拟语气：
①用于表示建议、命令的动词的宾语从句中。表示建议、命令的动词有suggest（表“建议”时）, advise（忠告）, recommend（推荐）, order（命令）, demand(要求）urge（力劝）,require(要求）, request(请求）,desire（渴望）, ask / insist（表“坚决要求”含义时）等词的宾语从句的谓语应由“should+动词原形”构成，此时should可省略。 He suggested that I（should）do the Job.
②用于①项中动词的名词形式以及wish, instruction, order, plan, idea等的表语从句或同位语从句中。
It is my wish that you（should）do so.
③用于某些主语从句中。
It is demanded we（should）get everything ready by the end of the week.
④should表惊奇、怀疑、不满、意外等，常用于“Why/How should...”句型和expect, think, imagine, believe等词的宾语从句中作“竟，竟然”解。 It's a pity that he (should) be so careless. 真可惜他竟然如此粗心。
⑤在if, lest, for hear that, in case引起的从句中，谓语用should +动词原形, 把可能性减弱。（译作“万一”，“要是真的”）。
If you should decide to accept, please phone me. =Should you decide to accept,please phone me. 万一你决定接受，请打电话给我。
⑥should(ought to)+ have done表“过去该做某事，却没做”，含有“责备、内疚”意。 She should (ought to) have helped you a lot.
⑦shouldn't (ought not to) + have done. 表做了不该做的事情，含有责备、后悔意。
（七）ought to
1. ought to表示“应该”，语气比should 强。 We ought to help each other in our work.
2. ought to表示因责任、义务等该做的事，具客观性。 should多表示某件事宜于做，带有一定的主观意见。 You're my father. You ought to look after me.
3. ought to表根据事实非常可能的事，意为“理应...”。 He is her neighbor, so he ought to know her address. 4. ought (not) to have done =should (not) have done。
（八）will
①表示意愿、意志和决心，用于各种人称。 I'll do my best.
②表示各种倾向或习惯动作。 Fish will die out of water. -Who can close the door? -I will（I'll do it）我来吧！
本句难点是：不能用I'm going to do it .或I'm to do it.而只能用will，因为本句话表反射性动作，不表计划、打算。
③表示一种揣想，用于二、三人称。 They will have arrived by now.他们大概已经到了。
④用于疑问句中，表邀请、请求。 Will you give me a piece of paper?
⑤will 在表示经常性、习惯性的含义时，有时可理解为“难免；总是”。 These things will happen.这种事难免要发生。 Boys will be boys.男孩毕竟是男孩。 Accidents will happen.事故难免会发生。
⑥表猜测时，表“想必…”，would的可能性要比will小。
（九）would would是will的过去式，would + v.构成过去将来时。除此之外，它还是单独的情态动词。
①表示过去时间的“意志”、“愿望”和“决心”。 I told him that I would go along with him.
②委婉提出请求、建议或看法, 比will委婉。肯定答语用will。 Would you help him? Yes, I will.
③表示过去的习惯动作和过去某种倾向。 When he was a child he would（used to）play football.
would与used to的区别
① used to主要用于今昔习惯的对比，表明过去如何而现在已不再如此。而would只表示过去习惯，不暗示现在。
②used to用于表过去的习惯动作或过去一贯存在的情况、状态，would则表过去习惯或倾向性的动作，不能表情况或状态。故used to接动作动词（work，stay等）也可接非动作动词（like, have等），would只接动作动词。 There used to be a house here.
④would用于固定习语中 would like sth.
would like to do sth.would rather +动词原形（than + v原形） I would rather do it myself. I would like to help you.
He'd rather die than let me think he was a failure. 他宁愿死也不愿让我认为他是个失败者。 ●would rather…than…
（十）need 需要, need既可做实义动作，又可作情态动词。
1. need作情态动词主要用于疑问句、否定句，否定式为needn't. 没有过去式形式，表达过去意义时仍用need。
Need you go soon?We needn't hurry. Need I come? No, you needn't (don't have to）/Yes, you must.注意
need问句的肯定回答用must，否定回答用needn't或don't have to.
2.用在有否定意思的句子中。（带有no, hardly等否定词的句子）。 I hardly need say how much I missed you.
3. need作实义动词有人称、数、时态的变化。肯定式：need to do。否定式：don't need to do。 注意
need doing = need to be done Does he need to know it? The windows need cleaning.
4. needn't have done表示“本不必做…（却做了）”。 I needn't have bought the book. You needn't have told them about it.
（十一）dare “敢”，既是实义动词，又是情态动词，用法如下：
1. dare作情态动词主要用于疑问、否定及条件句中，后接动词原形。没有人称和数的变化，但有时态变化，过去式为dared。
Dare you gohome alone? He dare not tell the truth.
2. dare作实义动词时，后跟不定式，用于肯定、否定、疑问等各种结构。有人称、数、时态变化，过去式为dared。
3.用于固定习语: “I dare say”意思是“我敢说，我认为”。 I dare say you are my friend.
情态动词表推测的用法归纳
1.“情态动词+动词原形”表示对通常或现在的行为或情况的推测。 He must know you.他一定了解你。 She may live here.她可能住在这里。
2.“情态动词 + be doing”用于对可能正在发生的动作的推测，“应当正在…，可能正在...”
He can't be reading. They must be waiting for you.
①此结构间或可表示“即将，将要”这类意思。 I must be leaving now.
3.“情态动词 + have done”结构表示对已经发生的事情的推测、责怪、后悔、遗憾等。
① must have done，表示对过去事情把握极大的推测，意为“必定已经”； “may（might) have done”为把握不大的猜测“可能已经”。
注意must表示推测时，通常只用肯定句中。 否定句为can't（couldn't）have done 疑问句can (could)... have done... ?
I didn't hear the telephone, so I must have been asleep. He can't have studied English before.
② may (might) have done还可表责备，“本来可以…”。
could have done表示“本能够做某事而没做”，指能力因素或可能性，可表示站在现在角度看过去（较委婉）也可指站在过去角度看过去的过去。
He might have given you more help, even though he was very busy. 他本应可以给你更多帮助，尽管他非常忙。
He could have walked to the station, it was so near. 我们本可以走着去火车站，这么近。
③should (ought to) have done表示“本该做某事，但实际上没做” should not（ought not to) have done表示“本不该做某事却做了”
The plant is dead. I should have given it more water. （本该多浇水，而实际没有）
Tom ought not to have told me your secret. 汤姆本不该把你的秘密告诉我（但他告诉了）。
④needn't have done.本不必做…（但实际却做了）（didn't need to do表示本不必做而实际上也没有做）。
I needn't have watered the flowers, for it's going to rain.
⑤will/would have done可表示推测，也可表示与过去事实相反。
They will have arrived by now. 他们大概已经到了。
The car would have been out of control if he had not reacted quickly.要不是他反应快，车子就会失控。
