情态动词,概说： 情态动词表示说话人的语气和情态，需要，可能，愿意和怀疑等。情态动词有一定的词义，但不完整，必须和主要动词的原形一起构成谓语。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, etc.,一. can , could, be able to I. 1) can 表示体力或脑力方面的能力，或根据客观条件能够做某事的能力。 A blind man can not judge color. He can speak five languages. 2) 表示许可，在疑问句中表示要求，否定句中 表示不许。 Can (May) I come in ? - Can I use your dictionary? - Of course, you can. This kind of thing cant go on.,3) 表示可能性，常用于疑问句和否定句中，指“某事是否是事实”。 Can it be true? Can the hall seat a thousand people? Someone is knocking at the door. Who can it be? She cant be here. He cant be serious. Mary cant have gone there alone. 4) 表示温和的命令或批评 You can go and fetch some water. You can clean the windows first. You could read more in future. You could be more careful.,5) be able to与can 的比较 A) 表示能力时可通用 No one can / is able to do it. Can you come tonight? /Will you be able to come tonight? B) be able to可用于任何时态， can只能用在现在时或过去时（ could ）中。 Im sorry I havent been able to answer your letter. He says / said he will / would be able to come home for Christmas. C) 表示经过努而成功地办到了某个具体的事情时，只能用be able to 不可用 can。 With the help of the firemen, they were able to leave the burning house. After years of hard work he was able to win the prize,2. could 1) could 是can的过去时，除具有can的各种功能外，还可以用来比较委婉，客气地提出问题或陈述看法。 Could you come a little earlier? Im sorry I couldnt lend you the book now. - Could I borrow your bicycle? - Yes, of course, you can. ( 不可用could ) I could come earlier, if necessary. 2) can 和could 还可表示某人或某物一时的特点，可译为“有可能，有时会”。 He can be very friendly. He could be very proud. Man can be as cruel as wild beasts. Children could be very trying. (伤脑筋),二. may, might 1） 表示许可，允许，译为“可以”（正式场合） You may take the book home. People may not pick flowers in the park. Id like to ask a question if I may ask. - May I watch TV tonight? -Yes, you may. (No, you mustnt/youd better not.) - May I smoke here? - Yes, please. / please dont.,2) 表示推测，“或许，可能” 表示一件事或许会发生，一般用在陈述句中。 It may rain tomorrow. They may not be there today. Fools may ask questions more than wise men can answer. Anybody may get ill. 注：may 指事实上的可能性, can 指逻辑上的可能性 Mr. Smith looks pale. He may be ill. Mr. Smith is in poor health. He can be ill at any time. 3) 表示祝愿 May our friendship live long! May you succeed! May you have a good time! May you be happy!,4) 常用在目的状语从句或让步状语从句中 Get up early so that we may catch the first bus. She went by air that she might /could arrive earlier. Whoever he may be, he should obey the rules. He would work hard, however rich he might be. 5）may well + 动词原形意为“理应，有足够的理由” may /might as well+ 动词原形用来建议或劝说某人采取某种行动，有时相当于had better常译为 “还不如,不妨”。 He may well be proud of his son. （他大可以他的儿子为荣） She may well say so. （她说得对） It is very late, so you may / might as well go to bed.,Might: might是 may 的过去式，在表示可能时可以换用，但might 可能性较小，或表示更婉转的语气。 Jim may lend you money. Jim might lend you money. Might I ask a question? Might I ask for a photo of your baby? I wonder if I might borrow some money?,三. must, have to, need 1. must 的用法： 1） 表示必须要做的事，意为“必须” ，否定式表示不应该，不许可禁止等。在回答must的问句时,不用mustnt,而用neednt或dont have to I must leave at 9. We must do everything step by step. We mustnt waste our time. I felt I must call and see him. - Must we hand in our exercises today? - Yes, you must. / No, you neednt. / No, you dont have to.,2) must表示“偏要，偏偏”，或“（干吗）硬要（多指不愉快的事） Why must you always interrupt me? Must you go so soon? The car must break down just when we were about to start off. After I give her advice, she must go and do the opposite. 3) must也可表示客观必然性，意为“必然会，总是会” 。 All men must die. Truth must be out. Winter must be followed by spring,3) must 还可表示一种推断和揣测。must +do对现在事实的猜测;must +have done 对过去事实的猜测. This must be Jims pen. You must be joking. I cant find my key. I must have left it in the bus. The book must have been written by a woman. You must have been thinking of something.,2. must与 have to的比较： 1). have to比较强调客观需要，表示因客观环境或事态促使而不得不做某事；must强调主观看法，表示主观上认为有必要做某事。 I must learn another foreign language. You have to learn another foreign language if you want to work here. You must be back before 10 oclock. You have to be back before 10 oclock because the train is to leave at 10: 15. 2) have to多表示义务或习惯动作； must 则表示一种重要或急迫的事情。 You have to care for the young. She has to be at the office before eight every day. You must go to the manager at once, or youll be dismissed.,3） have to可用于不同时态，可同其他情态动词 连用，而must则只有一种形式. We will have to buy another TV set. She is always having to make decisions. He has had to reconsider his position. She may have to stay there longer. I have to /must leave now.,3. need 的用法： 1）作情态动词用时，多用于疑问或否定句中。回答need提出的问句时，肯定常用must否定常用neednt / dont have to You need not do anything here. He never need know. She need hardly say anything to him. I dont think he need come. Need I repeat it? There need be no hurry, need there? - Need I go so soon? - Yes, you must. / No, you neednt. /