【易混辨析】：情态动词用法的比较.doc

【易混辨析】：情态动词用法的比较情态动词：can(could), may(might), must(must),have to, shall(should), will(would), need(needed), dare(dared), ought to。情态动词无人称和数的变化，不能独立使用。它与其后边的动词原形构成谓语。情态动词的具体用法如下：1.can 和could的用法（1）表示能力或客观可能性，还可以表示请求和允许。Can I go now? Yes, you can.Man cannot live without air.Can you finish this work tonight?注意：could也可表示请求，语气较委婉，主要用于疑问句，不可用于肯定句，答语应用can。Could I come to see you tomorrow?Yes, you can.（否定答语可用：No, Im afraid not.）can表示能力时，还可用be able to代替。Ill not be able to come this afternoon.但当我们要表示“某件事已成功”时，应用“was(were) able to”，而不能用“could”。He was able to go to the party yesterday evening and he enjoyed himself very much.（2）表示惊异、怀疑、不相信的态度。（主要用在否定句、疑问句或感叹句中。）This cannot be done by him.How can you be so careless!Can this be true?2.may和might的用法（1）表示许可。表示请求，允许时，might比may的语气委婉。否定回答时可用cant或mustnt，表示“不可以”、“禁止”、“阻止”之意。May I take this book out?Yes, you can. -No, you cant (mustnt)Might I use your pen? No, you mustnt.用May I 征询对方许可，在文体上比较正式，在口气上比较客气。在日常口语中，特别在现代口语中，用Can I 征询对方意见更为常见。（2）用于祈使句中表示祝愿。May you succeed!（3）表示推测、可能（不用于疑问句）。He may be very busy now.3.must和have to的用法（1）表示必须、必要。You must come in time.回答must引出的问句时，如果是否定的回答，不能用mustnt，而要用neednt或dont have to。Must we hand in our exercise books today?Yes, you must. / No, you dont have to.(you neednt)（2）“must be + 表语”的结构表示推测，它的否定或疑问形式用can代替must。This must be you pen.（3）must和have to表示“必须”时，意思很相似，但有下列几点区别应注意：must表示的是说话人的主观看法，而have to则往往强调客观需要。The play is not interesting, I really must go now.I had to work when I was your age.must一般只表现在，have to则有更多的时态形式。must可以表示推断，而have to则不能。询问对方的意愿时应用must。Must I clean all the rooms?Yes, you must .(No, you neednt.)二者的否定意义大不相同，must not 表示禁止，是说话人强有力的劝告，意为“决不可以”、“不准”、“不允许”。dont have to意为“不必”。You mustnt go. 你决不不可去。You dont have to go. 你不必去。4.dare和need的用法（1）dare作情态动词时，主要用于疑问句、否定句和条件从句中，一般不用于肯定句。How dare you say Im unfair!He darent speak English before such a crowd, dare he?（2）need表示“需要”或“必须”，作情态动词用时，仅用于否定句或疑问句中。在肯定句中一般用must, have to, ought to, 或should代替。You neednt come so early.Need I finish the work today?-Yes, you must.（3）dare和need常用作实义动词，有时态、人称和数的变化。所不同的是，作实义动词时，在肯定句中，dare后面通常接带to的不定式；在否定句和疑问句中，dare后面可接带to或不带to的不定式。He needs to finish it this evening.I wondered he dared (to) say that.Dont you dare (to) touch it!He does not dare (to) answer.I dare to swim across this river.5.shall和should的用法（1）shall用于第一人称，表示征求对方的意愿。What shall we do this evening?（2）shall用于第一、第三人称疑问句中，表示说话人的征求对方的意见或向对方请求。When shall he be able to leave the hospital?Shall we begin our lesson?（3）shall用于第二、第三人称，表示说话人给对方的命令、警告、允诺或威胁。He shall be punished. （威胁）He shall have the book when I finish reading.（允诺）You shall fail if you dont work harder. （警告）（4）should表示劝告、建议、命令。其同义词是ought to。在疑问句中，通常用should代替ought to。Should I open the window?You should go to class right away.6.will和would的用法（1）表示请求，建议等，would比will委婉客气。Would you pass me the book?（2）表示意志、愿望和决心。They asked us we would do that again.I will never do that again.（3）would可表示过去反复发生的动作或某些倾向。would表过去习惯时比used to正式，且没有“现已无此习惯”的含义。During the vacation he would visit me every week.The wound would not heal.（4）表示估计或猜想。I thought he would have told you all about it.What would she be doing there?It would be about ten when he left home.7.ought to的用法（1）ought to表示“应该”（和should差不多，只是口气稍重一些），无人称或时态变化。You ought to take care of him.注意：在美国英语中，ought to用于否定句和疑问句时，to可以省略。You oughtnt smoke so much.Ought you smoke so much?（2）表示推测。注意与must表示推测时的区别：He must be home by now.（断定他已到家）He ought to be home by now.（不十分肯定）This is where the oil must be.（比较直率）This is where the oil ought to be.（比较含蓄）8.“情态动词 + 不定式完成式”的用法（1）“can（could） + 不定式的完成式”的肯定式表示“本来可以做而实际上未能做某事”，疑问或否定形式表示对过去发生的行为怀疑或不肯定。Can he have got the book?He cannot have been to that town.You could have done better, but you didnt try your best.（2）“may（might） + 不定式的完成式”表示对过去发生的行为的推测。If we had taken the other road, we might have arrived earlier.He may not have finished the work.might所表示的可能性比may弱，语气委婉。（3）“must + 不定式完成式”的结构常用在肯定句中，表示对过去发生的行为的推测，意为“一定，想必”，它的否定或疑问形式用can代替must。He must have been to Shanghai.You must have seen the film The Gold Rush.（4）“neednt + 不定式的完成式”表示本来不必做某事而实际上做了某事。You neednt have watered the flowers, for it is going to rain.注意：didnt need to (have to) do指“没有必要做而实际上也没有做某事”。I didnt need to clean the windows. My sisiter did it.（5）“should + 不定式的完成式”表示本应该做到而实际上没有做到。其否定式表示某种行为不该发生但却发生了。She shouldnt have taken away my measuring tape, for I wanted to use it.You should have started earlier, but you didnt.注意：He should have finished the work by now.（表推测）（6）“will + 不定式的完成式”的结构表示推测，主要用于第二、三人称，表示对已经完成的动作或事态的推测。He will have arrived by now.（7）“ought to + 不定式的完成式”表示过去应做某事而实际未做。You ought to have helped him (but you didnt).这时，ought to与should可以互相换用。9.used to，had better， would rather的用法（1）used to表示过去的习惯动作或状态（现在已不复存在），在间接引语中，其形式可不变。He told us he used to play football when he was young.在疑问句、否定句、否定疑问句或强调句中，可有两种形式：疑问句：Did you use to go to the same school as your brother?Used you to go to the same school as your brother?否定句：I usednt to go there.I didnt use to go there.其反意疑问句或简略回答中，也有两种形式：She used to be very fat, didnt she?/ use(d)nt she?Did you use to play chess? Yes, I did.Used you to get up early in the morning? Yes, I did./ Yes, I used to.（2）had better意为“最好”，后接不带to的不定式。You had better have done that.（用于完成时态，表示未完成动作。）I think Id better be going.（用于进行时态，表示“最好立即”。）注：had best与had better同义，但较少用。You had b