四．情态动词情态动词体现说话者的语气、情感、态度但本身词义不完全，不能单独作谓语动词，必须和不带to的不定式(即动词原形)连用情态动词没有人称和数的变化，但有的有过去式情态动词数量不多，但其用法灵活多变，相互间区分度小，不易把握，需在熟练掌握其在各种句式中的基本含义和用法的基础上加以细心体会、区别A．情态动词can，could，may，might，will，would，shall，should，must，can’t等的用法1. can和could 1) 表示“能够做……” e.g. No one could pass the exam last week.2) 表示许可 e.g. You can stay here until your father comes to pick you up.3) 表示请求，这时，could比can的语气要委婉e.g. Could you lend me a pen, please?4) 表示可能、猜测常用于否定句和疑问句；有时也用于肯定句，表示的是逻辑上的可能性 e.g. He can’t be at home at this time. Mr. Smith is in poor health, and he can be ill at any time.★ can和be able to的区别：1) 形式上的区别：can只有现在式和过去式could；而be able to则可用于多种时态。
如：is able to，were able to，will be able to等2) 语义上的区别：can通常用来指一种固有的能力，如：Ducks can swim. 鸭子会游水而be able to则常用来指某一次具体的行为，且是通过了努力才能够做成某事 e.g. Though completely drunk last night, he was able to find his home. 虽然昨晚他完全喝醉了，但他还是可以找到家3) could和was able to :在肯定句中，表示特定的某一过去能力要用was／were able to，但否定句中could不受这一限制 e.g. He was very ill, so he couldn’t／wasn’t able to finish the work on time.2. may和might 1) 表示许可，可与can互换2) 表示请求，语气较委婉3) 表示可能性，但说话人把握不大（若有确切把握且在肯定句中则用must，否定句中用can’t）通常只用于肯定句或否定句在肯定句中表示的是事实上的可能性。
e.g. John looks weak. He may be ill.3. will和would 1) 两者都可以表示承诺和意愿 e.g. The whole family will come for dinner. She asked if I would help.2) 表示请求，此时用would语气较委婉e.g. Would you join us?3) would还常用来表示过去的习惯和传统 e.g. He would sit alone for a whole morning when he was upset during his childhood.4. shall和should 1) 表示承诺和意愿，但常用于第一人称shall用于第二、三人称时，表示说话者的意愿、意志，意思是“应、要、必须” e.g. I promise that she shall get a nice present on her birthday.2) 表示建议 e.g. You should learn more about western culture if you want to study abroad.3) 表示预测“应当”。
e.g. It’s nearly 7 o’clock. Dad should come home soon.5. must和can’t两者都可以表示推测，must表示肯定的推测，用于肯定句中can’t表示否定的推测，常用于否定句中e.g. They must have got lost. 他们一定是迷路了6. need 1) need作情态动词时无人称和时态变化意思是：需要 2) 肯定回答用must，否定回答用needn’t 一Need I go with you? 一Yes, you must. / 一No, you needn’t.¹ SELF ASSESSMENTSI. 单句改错1. Mr. Smith looks pale. He can be ill.2. When we worked in the same office, we will often have coffee together.3. You must be joking. That mustn’t be true.4. Come to take the photos tomorrow morning. It may be ready by then.5. The fire spread so fast that only one-third of the people could escape the fire.II. 用正确的情态动词填空。
1. When we are cooking with a gas fire, we ____________ (can／must) keep the window open.2. Mr. Joseph has stayed in the US for several years. He ___________ (may／can) speak English quite well.3. No one ____________ (will／would) think highly of a woman in the old times.4. It ______________ (can’t／mustn’t) be Miss Gao. I know she has gone to Beijing.5. You say you _____________ not do it but I say you _____________ (shall／will).III. 单项填空1. 一Isn’t that Ann’s husband over there?一No, ___________ be him -- I’m sure he doesn’t wear glasses. (2004全国)A. can’t B. must not C. won’t D. may not2. “The interest ______ be divided into 5 parts, according to the agreement made by both sides.” declared the judge. A. can’t B. should C. must D. shall3. The fire spread through the hotel very quickly but everyone ___________ get out. (1997全国) A. had to B. would C. could D. was able to4. 一Will you stay for lunch? (1999全国) 一Sorry, ____________. My brother is coming to see me. A. I mustn’t B. I can’t C. I needn’t D. I won’t5. 一Write to me when you get home. (2001北京春) 一 _______________. A. I must B. I should C. I will D. I can6. – Are you coming to Jeff’s party? -- I’m not sure. I ___________ go to the concert instead.A. must B. would C. should D. might7. – Is John coming by train? -- He should, but he _________ not. He likes driving his car.A. must B. can C. need D. may8. A left-luggage office is a place where bags __________ be left for a short time, especially at a railway station. A. should B. can C. must D. will9. – Shall I tell John about it? -- No, you ____________. I’ve already told him. A. needn’t B. wouldn’t C. mustn’t D. shouldn’t10. Tom, you ____________ leave all your clothes on the floor like this.A. wouldn’t B. mustn’t C. needn’t D. may notB．情态动词ought to的用法1. ought (not) to，should及have to表示“应该”：情态动词用法举例ought to“应该”，多指客观上有责任，义务干某事，语气比should强烈。
否定形式：ought not to (oughtn’t to) ＋ V-原① You ought to start at once.② Such things ought not to be allowed, ought they?should“应该”，多指主观上有责任，有义务干某事，语气比ought to弱 否定形式：should not (shouldn’t)① We should memorize more words.② Everyone should know some first aid.have to“应该，不得不”，多指客观形。