4. can 和 may
考试中主要测试can，may或could，might表示可能性的区别及对may构成的疑问句的回答
can, could, may, might都可以表示可能性.can, could表示潜在的可能性或理论上或逻辑判断上存在的可能性；而may, might则表示事实上的可能性。此外，can还具有“有能力”的意思，而may与might则不具此意。例如：^
① According to the weather forecast, it may rain tomorrow. 据气象预报，明天可能下雨。
② Any man with a little sense can see that he is wrong. 任何一位有头脑的人都能看出他错了。
May I / we …？这一类疑问句的肯定回答为Yes, please.或Certainly；否定回答为Please don’t.或No, you mustn’t. 例如：
③ ---May we leave now? 我们可以走了吗？
---No, you mustn’t. You haven’t finished your home work yet. 不行，你们还没完成工作呢。
3. would和used to
used to表示过去与现在或过去某时与后来的情况有不同，而would只表过去的情况。例如：
① People used to think that the earth was flat.过去人们认为地球是平的。(现在人们不这么认为。)
② She would go out for a walk in the morning when she was in the country.
在乡下时，她总是在早晨去散会儿步。(可能现在仍有散步的习惯。)
used to可表示过去的习惯动作和经常的情况，而would只表示过去的习惯动作。例如：
① He used to ∕would smoke while writing. 过去他写东西时常抽烟。
② She used to be fat. 她过去很胖。
2. must和 have to
must 和 have to 都可以表示“必须”，但有几点区别：
must 强调“内在的职责”、“义务”，而have to 强调“外界压力”、“不得已而为之”。^
have to可用于多种时态，而must一般用于现在时，其过去时与将来时分别由had to与shall / will have to代替。
在回答must引起的问题时，如果是否定的答复，不能用mustn’t，而要用needn’t或don’t have to，因为mustn’t是“一定不要”、“一定不能”的意思。例如：
① You must come to the classroom before eight. 八点前你必须来教室。
② It is raining hard outside, but I have to leave now. 虽然外面下着大雨，可我还是得走了。
③ ---Must we do it now? 我们必须现在做吗？
---No, you needn’t. 不，不必。
1. can 和be able to
情态动词can只有两种时态形式，现在式can和过去式could，而be able to有多种时态形式。例如：
① Mary can play the piano. She has been able to play it since she was 5. 玛丽会弹钢琴。她五岁起就会弹了。
用在过去时中，could经常表示能够做某事，事实上不一定去做，而was / were able to则表示“过去做成了某事”。在否定句中两者可通用。例如：
② He could swim across the English Channel. But he didn’t feel like it that day. 他能游过英吉利海峡，但那天他不想游。
③ Yesterday I was able to get home before the heavy rain. 昨天我在下大雨前赶到了家里。
1)used to表示过去与现在或过去某时与后来的情况有不同，而would只表过去的情况。
People used to think that the earth was flat.
过去人们认为地球是平的。(现在人们不这么认为。)
She would go out for a walk in the morning when she was in the country.
2)used to可表示过去的习惯动作和经常的情况，而would只表示过去的习惯动作。
He used to ∕would smoke while writing. 过去他写东西时常抽烟。
She used to be fat. 她过去很胖。
◆ 考题剖析
例题1. I didn't hear the phone. I ________ asleep. (NMET 89)
A. must be B. must have been C. should be D. should have been;
答案 B 从原题中的I didn't hear the phone．这一信息可以判断出，用must have
done表示对过去事实的准确的肯定猜测。
例题2. Tom ought not to me your secret ,but he meant no harm. (NMET93)
A. have told B. tell C. be telling D. having told
答案 A 该句意思是：“汤姆本来不该告诉我你的秘密，但他没有伤害你的意思。”
例题3. I was really anxious about you. you home without a word. (NMET2001)
A. mustn't leave B. shouldn't have left
C. couldn't have left D. needn't leave.
答案 B shouldn't have done为不该做了某事。
例题4. ---Could I borrow your umbrella, please?
---I’m afraid you _______.
A. could B. can C. couldn’t D. can’t
答案 D 此题极易错选C。其实could在问句中表示请求，为了语气的委婉和客气才用过去式，而答句必须按照实际情况回答，故选can’t，以求和I’m afraid(在此表示婉拒)一致。
例题5. ---Don't forget to come to my birthday party tomorrow.
---______.`
A. I don't B. I won't C. I can't D. I haven't
答案 B will既可当作情态动词，表请求、建议、也可作为实义动词表"意愿、意志、决心"，本题表示决心，故选B。
例题6. ----Shall we go skating or stay at home?
----Which ___ do?
A. do you rather B. would you rather C. will you rather D. should you rather
答案 B 本题考查情态动词would rather的用法，would rather +do sth 意为“宁愿”，本题为疑问句，would 提前，所以选B。
◆思考
几组情态动词的辨异
2. must和 have to语法学习纲要由江苏省靖江高级中学外语组为英语猫网站特供
must表示主观意志，而have to表示由于客观因素不得不做某事。must没有过去式，除在间接引语中可用于表示过去时间，在直接引语中表示过去时间应该用had to代替。
I told her that she must give up smoking. 我叫她必须戒烟。
We had to get everything ready that night. 我们那晚得把一切准备就绪。"
1)情态动词can只有两种时态形式，现在式can和过去式could，而be able to有多种时态形式
Mary can play the piano. She has been able to play it since she was 5.
玛丽会弹钢琴。她五岁起就会弹了。
2)用在过去时中，could经常表示能够做某事，事实上不一定去做，而was / were able to则表示“过去做成了某事”。在否定句中两者可通用。
He could swim across the English Channel. But he didn’t feel like it that day.
他能游过英吉利海峡，但那天他不想游。
Yesterday I was able to get home before the heavy rain. 昨天我在下大雨前赶到了家里。
1. 情态动词后跟完成式和进行式的用法
1) 情态动词+动词完成式
情态动词+动词完成式即“情态动词+ have + done分词”，表示对过去行为或动作进行推测、评论或判断。
must have done 表示对过去某事的肯定猜测。其否定或疑问形式都用can(could) 来表示。 例如：
① Since the road is wet, it must have rained last night.
② He can’t have missed the way. I drew him a map.
③ The dictionary has disappeared. Who could have taken it?
当然对现在发生或将来发生的事情,要用must do 表示猜测, 否定为can’t do。例如：
④ He must understand that we mean business.
⑤ You must be hungry after a long walk.
may / might have done表示推测过去某事“也许”发生了.may 比 might 表示的可能性在说话人看来稍大些。例如：
⑥ I can’t find my keys. I may / might have left them at the school yesterday.
could have done 在肯定句型中, 往往用作委婉的批评. 本应该做什么,而没做; 有时也用作猜测。例如：
⑦ You could have told us earlier.
⑧ Tom could have taken the dictionary.
ought to / should have done和 ought not to / shouldn’t have done用于对已发生的情况表示“责备”、“不满”，分别表示“本应该…”和“本不应该…”。例如：
⑨ With all the work finished, I should have gone to the party last night.
⑩ You ought not to have made fun of him. He is not the one you laugh at but learn from.
needn’t have done 表示过去做了某事，但没有做的必要， 意为“本没必要…”。例如：
⑾ You needn’t have waken me up; I don’t have to go to work today.
[注意]表示推测过去某动作发生的可能性时，就表示的可能性程度而言，must最大，could其次，may更次之，might最小。例如：
⑿ ---I wonder how Tom knew about your past.
---He must / could / may / might have heard of it from Mary.
2) 情态动词+动词进行式
情态动词+行为动词进行式(即情态动词+ be + doing形式)，表示推测或评论某动作现在是否正在进行。例如：
① He must be playing basketball in the room.
② She may be staying at home.
3) 情态动词+动词完成进行式
情态动词+行为动词完成进行式(即情态动词+ have been + v-ing形式)，表示推测或评论过去某动作是否正在进行或一直在进行。例如：
① They should have been meeting to discuss the problem.
② He may / might have been buying stamps in the post office when you saw him.|
9.特殊情态动词need和dare的用法：
need和dare既可用作情态动词，也可用作实义动词。用作情态动词时，主要用于否定句和疑问句。用作实义动词时，可用于各种句式。
1) 用作情态动词。例如：|
① You needn’t telephone him now. 你现在不必打电话给他。
② I don’t think you need worry. 我想你不必发愁。
③ She dare not go out alone at night. 她晚上不敢一个人出去。
④ How dare you say I’m unfair? 你竟敢说我不公平？_
2) 用作实义动词。例如：
① You don’t need to do it yourself. 你不必亲自做这件事。
② We need to tell them the news. 我们需要把这消息告诉他们。
③ The table needs painting (to be painted.). 桌子需要油漆一下。
④ We should dare to give our own opinion. 我们要敢于提出自己的观点。
⑤ He did not dare (to) look up. 他不敢抬头看。
⑥ I dare say he’ll come again. 我想他会再来的。
[注意](I dare say…为固定习语)
◆ 难点突破
8. used to
表示过去的习惯动作或状态，现在不复发生或存在。疑问式和否定式有两种。例如：i
① There used to be a building at the street corner, but it has been pulled down. 街道拐角处过去有座楼房，现在拆了。
② I usedn’t (didn’t use) to smoke. 我过去不抽烟。j }F
③ Used you (Did you use) to go to school on foot? 你过去常步行去学校吗？