Modal Verbs情态动词有can (could), may (might), must (must), have to, shall (should), will (would), need (need), dare (dared), ought to 等。
情态动词无人称和数的变化。
它不能单独使用，必须跟其后的动词原形构成谓语。
情态动词的具体用法如下：一、can, could1、表示能力。
a. Can you speak English?b. Can you finish this work tonight?c. Man cannot live without air. Note:(1) can表示能力时，可用be able to 代替。
a. I’ll not be able to come this afternoon.(2) 当我们要表示“某件事情已经成功”时，应用was/ were able to , 不能用coulda. He saw well and he was able to swim to the river when the flood happened.b. He was able to go to the party yesterday evening and he enjoyed himself very much.2、表示客观可能性a. People who live near airports can have their hearing harmed.b. The boy can sometimes be very naughty. （表示某人或某物一时的情况，“有时会。
”）c. The invention can be improved.d. Even experts can make mistakes.e. He can’t be in Beijing now because I saw him a moment ago.f. He had a lot of work to do last night, so he couldn’t have gone to see a film.Note:在肯定句中用can表示可能性时，其含义是理论上的可能性，不表示实际的可能性，也就是说，不涉及到是否真会发生。
3、表示惊异、怀疑、不相信的态度（主要用与否定句、疑问句或感叹句中）a. Can this be true?b. How can you be so careless!c. This cannot be done by him.4、表示请求和允许。
a. --- Can I go now?--- Yes, you can.b. You can smoke if you want to.c. You can’t pick flowers in this park. Note:could也可以表示请求，语气较委婉，主要用于疑问句，不用于肯定句，回答应用can--- Could I come to see you tomorrow?--- Yes, you can. / No, I’m afraid not.5、can 的一些其他惯用法(1). cannot but, cannot help but, cannot choose but +动词原形，意思为“不得不，只得”。
a. I cannot help but tell her the truth.2. cannot help doing stha. I cannot help laughing.3. cannot …too / enough, can never tooa. You cannot be too careful when you cross the street.b. I cannot thank you too much for your kindness; I owe my progress to you.二、may, might1、表示许可。
a. You may go now.b. We may keep the books for two weeks.c. --- May I smoke here?--- Yes, you may. / No, you may not. / No, you mustn’t.2、表示可能性a. He may be waiting for you at the station.b. They may have got lost.Note:表示可能性时，may 不用于疑问句。
在疑问句中表示“可能”时，通常用“be likely to ”或“Do you think”这一结构。
3、用于让步状语从句中a. However hard you may study, you cannot master English in a month.b. Don’t give up whatever difficulties you may meet with.4、用于祈使句，表示祝愿。
a. May you succeed!b. May we never forget each other!5、might常用于表示委婉的请求或轻微的责备。
a. You might post this letter for me if you are going near a post box.b. You might have let me know before!６、用于某些习惯用法中(1) may /might as well = had bettera. It’s getting darker and darker. You may/might as well go home.(2) may well + 动词原形，意思为“理所当然，有足够的理由”。
a. He has grown up tall. You may well not recognize him.三、must, have to１、表示义务和强烈的劝告。
意思为“必须”、“一定要”。
a. You must finish your task ahead of time.b. You must talk to your daughter about her future.2、表示有把握的推测，意思为“一定是”，“准是”。
a. I failed in the exam. You mustthink I am stupid.He must be a college student, isn’t he?b. You look happy. You must be having a good time.c. He looks tired. He must have stayed up l ate last night, didn’t he? d. You speak English so fluently. You must have learned it for many years, haven’t you?3、表示肯定性或难以避免，意思为“必然会、”“肯定会”。
a. All men must die.b. Don’t bet o horse races; you must lose in the long run.4、作“偏要”、“硬要”解。
（1）常以第二人称为主语，意思指不耐烦过令人不愉快的事情。
a. If you must smoke, at least you should do it outside.b. If you must know, I’m going to help him look for an apartment.c. Why must you buy that car? （2）用于其它人称，表示主语“固执”或“不巧”的意思。
a. The car must break down just as we were starting our holiday.b. Jane was never a pleasant young girl. After you gave her your advice, she must go and do the opposite.5、must的否定有如下三种形式，用于三种不同的场合。
(1) 当must表示“推测”时，其否定含义为“不可能”。
将mustbe 改为cannot be；将musthave done 改为cannot havedone。
a. It must be eleven o’clock now.It cannot be eleven o’clock.b. You must have met him before. You cannot have met him before.(2)当must 表示“必须”时，其否定含义为“不必”。
将must do改为need not do 或don’thave to do 。
a. You must get up at six tomorrow morning.You need not get up at sixtomorrow morning. / You don’thave to get up at six tomorrowmorning.（3）回答以must提问的句子，可以如下形式进行肯定或否定形式作答。
Yes, you must.a. Must we clean all the rooms? No, you don’t have to.No, you needn’t.6、must 和have to的区别四、ought to1、表示“应该”（与should同义，只是语气稍重一些, ought to 强调责任、义务），无人称和时态变化。
a. You are his father. You ought to take care of him.b. You ought not to smoke here.c. Ought you to smoke so much?五、shall, should1、在疑问句中，用于第一、第三人称，表示征求对方的意见。
a. Shall we do this evening?b. Shall I do it for you?c. Shall he come in or wait outside? = Do you want him to …?2、用于第二、第三人称，表示说话人给对方的命令、警告、允诺或威胁。
