情态动词和其他动词一起使用，表示愿望，态度或推测，没有人称和数的变化1. 表示能力 abilitycanI cant promise anything, but Ill do what I can. Dont worry about me. I can take care of myself. Pork can be roasted. be able to I am able to finish the task on my own. If you have a good sleep, you will be able to work out the problem. The fire spread through the hotel very quickly but everyone was able to get out. 2. 表示推测 possibility must/have to He must be the wanted man. Hes exactly like the one in this picture. He has to be the wanted man. should/ought toHe should be here on time because he started early enough. He ought to be here on time. There shouldnt be any difficulty in getting you a visa. can, 可能会, 也用于否定和疑问Accidents can happen on such rainy days. What can they be doing?Where can she have put it? Things cant be that bad. mayThat may be our taxi. We may go to the cinema tonight, but we are not sure yet. Dont worry. Your father may not have been hurt seriously. 3. 表示必要性， necessityI must give up smoking. You must cope with criticism. I have to be careful what you say on Tom. Freedom need not mean independenceAdults need to live their own lives and thats difficult with children.4. 表示义务，obligationI should go and visit him this afternoon, but I wonder if I will be free. 5. 表示请求、建议，request, suggestion, 多用于疑问句May I sit down?Shall the driver wait outside? Would you do me a favor? Could I use your bike tomorrow morning? Yes, you can. 6. 表示意愿 wish If you will, you can stay here as long as you can. I will say no more on these matters, important though they are. I would rather do some reading at home than go shopping with them.I prefer to do some reading at home rather than go shopping with them7. could have done/should have done/neednt have done/would have done 本可以，本应该，本不必, 宁愿做，实际相反I could have worked the problem, but I was too nervous. You neednt have worked that late last night. It was harmful to your health. You should have come to the conference yesterday. What was the reason for your absence? I raised objections at the meeting, but now I would rather not have done that.