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1、 专题八 情态动词 情态动词本身有一定意义，表允诺、愿望、义务、命令和猜测。它必须和动词原形一起构成谓语。情态动词没有人称和数的变化。把情态动词放在句首、句尾用问号便构成疑问句；在其后加上not或never等否定词就构成否定句。一、常见情态动词的用法（一）can/ could的用法1.表示具备某种“能力”，但不一定做事情，仅仅表示有能力而已。could是can的过去时：Can you swim across the river?你能游过那条河吗？2.表示“请求”（疑问句中）、“允许”。Could比Can委婉；两者都指现在。回答一律用can, may或of course,否定用mustnt。3.
2、表示“可能性”，都可以表示现在和将来，只是could的语气更加不肯定。否定式表示“不可能”，如：They cant/ couldnt be doing that now.（二）may/ might的用法1.表示允许或请求允许，might语气更委婉。肯定回答用may/ might/ can，否定回答用may not, cant, mustnt.May I turn on the TV?Yes, you may/ can.(No, you cant/ mustnt.)2.表示可能性，“也许”。这时may和might表现在，只是might在语气上更不肯定一些。如：You may/ might hav
3、e some fever.表示可能性时，may/ might不用于疑问句，可用can/could或别的说法。如：Is it likely to rain?3.祈使句中表示祝愿：May you succeed!（三）must的用法1.表示义务、命令或劝告，“必须”。You must tell me the truth.你必须和我说实话。在回答由must引起的问题时，否定回答用neednt或dont have to,表示“不必、没必要”；而mustnt表示“不准许”的意思。Must the ladies wear dresses?No, they dont have to / they needn
4、t.2.表示推测，“肯定”。此时，must只用于肯定句。在否定句或疑问句中，用can/ could：There must be a mistake. Can / Could there be a mistake?（四）need和dare的用法情态动词need“需要”和dare“敢”通常只用于否定句和疑问句中。You need not come so early. How dare you be so rude?但need和dare可作为实义动词，后跟动词不定式，可用在所有句型中，注意此时他们的否定式和疑问式与情态动词不同。（五）shall和will的用法1.shall用在第一、三人称作主语的疑
5、问句中，来征求意见。Shall I go now? Shall we invite her, too?2.shall用于所有人称，表示说话人的决心、保证、许诺、威胁、警告、命令等强烈的感情，如：You shall have an answer by tomorrow. He shall suffer for this.3.will可以表示“愿意、肯、会、固执”等意义，而非将来，would则更婉转：I will pay for it. Were going on a climbing trip. Come if you would.4.will可以表示倾向、习惯，在否定句中可以表示“不肯、不能”
6、等意思：Oil and water will not mix. This machine wont work.5.would可以表示过去的习惯，类似used to：He would come to see me on Sunday when he was here.（六）should / ought to的用法 两者大多数情况下可以互换。Ought to的否定式为ought not to / oughtnt to 。1.表示道义上的责任，义务或要求，有时表示劝告，“应该”，如：You ought to / should pay more attention to what your teach
7、er says.2.表示对现在和将来的推测：This pen ought to/ should be yours.3.“should/ ought to have+过去分词”表示对过去的推测，“应该已经”。有时表示“本应该做而没有做”；“should not/ ought not to have+过去分词”表示“做了不应该做的事情”，是“本不该”的意思，因而它们都含有责备的语气：He began two hours ago. He ought to have finished filling in the forms now. You should not have asked him for
8、 help.4.表示讲话人惊奇，失望，愤怒等感情：Why should I believe you? Whats happened to that money? How should I know?（七）had better表示“最好”，后接动词原形，其否定式为had better not+ 动词原形would rather意思是“宁愿”，表示选择。它有两种用法：一是在肯定句中后边直接跟动词原形，其否定句是在would rather后加not，即：would rather not+动词原形；二是它后边可以接从句，该从句的谓语动词用一般过去式表示虚拟语气。由于would rather表示选择，它
9、后边可接than。二、情态动词+ have+ done（一）must have done表示对过去的推测，意思是“一定已经，想必已经，准是已经”，只用于肯定句中。The streets are wet. It must have rained.街道是湿的，准是下雨了。否定要用“cant/ couldnt+ have+过去分词”，意思是“不可能”：The pocket cant have been lost there.（二）may/ might have done表示对过去的推测，意思是“也许已经 ，可能已经”。用于肯定或否定句中。疑问句中用can或could.此外，“might have+过
10、去分词”也表示“本来可以但没有实现”的意思，含有责备的口气。例如：He may have gone to bed. You might have told me earlier.你本可以早点告诉我的。（埋怨责备）（三）neednt have done“neednt +have+过去分词”表示“本来不需要做”，“做了不必做或不需要做的事”，如：I neednt have borrowed the money yesterday.注意didnt need to do表示“没有必要做而实际上也没有做某事”。（四）：can/ could have done:用来谈过去的情况1.在疑问句、否定句中，表示
11、对过去的行为的怀疑和不可能，这时它们没有时间上的差别，只是could的语气更弱一些：Who could have taken them? He cant have taken it home.2.could have done在肯定句中，表示“那时本来可以 ；差点就要”，有时用来进行婉转地批评：He could have told her, but he did not choose to.（五）should/ ought to have done表示“过去应该做某事却没有做”；“should not/ ought not to have+过去分词”表示“做了不应该做的事”。含有责备或遗憾的意
12、思：You should have come here ten minutes earlier.三、几组情态动词用法的区别（一）can和be able to的区别。在一般将来时和现在完成时中只能用be able to。can表示具备某种能力，但不一定做事；而be able to表示后天努力设法去做，成功后说明有能力，如：He was able to swim across the river in fifteen minutes.(= He managed to swim across the river in fifteen minutes.)（二）must与have tomust强调主观意
13、志，必要性来自说话者的主观意志，“必须”。Have to强调客观因素，“不得不”。如：He must stop smoking.他必须戒烟。（说话人认为他必须）You have to go and see the boss.你一定得去见见老板。（客观因素）注意：must not表示“绝对不可”；dont have to（或havent got to）表示“不必，没必要”，两者意思极为不同。（三）would与used towould可表示反复发生的动作或某种倾向。Used to表示过去的习惯动作或状态，强调现在已不存在。如：She used to work into the night.过去她常
14、工作到深夜。（现在不了）She would work into the might.以前她常工作到深夜。（不表示现在不了）【例题精析】例1. Tom, you _ leave all your clothes on the floor like this!A.wouldnt B.mustnt C.neednt D.may not答案：B 。例2. Where is my pen? I _ it.A. might lose B. need have lost C. must lose D. must have lost答案：D 。 专题测试1.Could I borrow your book?
15、Yes, of course you _ .A. may B. could C. must D. can2. I wonder how he that _ to his parents.A. dare to say B. dared say C. dares to say D. didnt dare to say3.Shall I tell him about it? No, you _ . I have told him already.A. neednt B. wouldnt C. shouldnt D. mustnt4. I _ have written to him because he phoned me later.A. ought to B. must C. couldnt D. neednt5.You can hardly swim, can you? _ . But my mother said
