1. can 和be able to
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.
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.
Yesterday I was able to get home before the heavy rain. 昨天我在下大雨前赶到了家里。
2. must和 have to
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。
3. would和used to
People used to think that the earth was flat.
She would go out for a walk in the morning when she was in the country.
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。
1. I __________ you，because I thought I must be wrong.
A. dare not ask B. dare not to ask C. dare not asking D. dare to not ask
2. There __________ some flowers in the garden.
A. were used to be B. used to be C. uses to be D. used to be having
3. “__________ I take it out？” “I'm sorry，you __________.”
A. Could …couldn't B. Might…might not C. Could…can D. May…can't
4. You were stupid to climb the tree. You __________ hurt yourself.
A. may B. might C. will D. might have
5. You __________ those letters. Why didn't you ？
A. should post B. should have posted C. must have posted D. ought to post.
6. All the lights are on，the Smiths __________ up.
A. must get B. is getting C. must be getting D. would get
7. He __________ lead a horse to the water but he __________ not make it drink.
A. will…can B. may…can C. may…dares D. dare…can
8. “Need we do this job now？” “Yes，__________.”
A. you need B. you should C. you must D. you can
9. __________ to have lunch with us today？
A. Do you likes B. Would you like C. Will you liked D. Have you liked
10. He said that you __________ watch TV all the evening if you wished.
A. may B. must C. can D. might
11. –Is John coming by train？ ——He should，but he __________ not. He likes driving his car.
A. must B. can C. need D. may
12. Peter __________ come with us tonight，but he isn't very sure yet.
A. must B. can C. may D. will
13. Michael __________ be a policeman，for he's much too short.
A. needn't B. can't C. shouldn't D. won't 14. I thought you __________ be hungry，so I have brought you some cakes.
A. may B. might C. can D. could
15. I wonder how he __________ that to the teacher.
A. dare to say B. dare saying C. not dare say D. dared say
16. Come on！ We __________ hurry because there isn't much time left.
A. may B. must C. can D. need
17. Amy did best in the English test. She __________ hard last week.
A. must have working B. should have worked C. should work D. must work
18. The fire spread through the hotel very quickly but everyone __________ get out.
A. had to B. would C. could D. was able to
19. ——I stayed at a hotel while in New York. ——Oh，did you？ You __________ with Barbara.
A. could have stayed B. could stay C. would stay D. must have stayed
20. It's nearly seven o'clock. Jack __________ be here at any moment.
A. must B. need C. should D. can