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情态动词表示说话人在能力、必要、义务或猜测等方面的语气或态度，有自已的词义，但不能单独作谓语动词，必须和动词原形一起构成谓语动词，只有情态动词ought要和带to的动词不定式连用,将情态动词置于主语之前即构成其疑问式，在情态动词之后加not既构成其否定式。没有人称和数的变化。如：can, may, must, would, should, need, dare, might, used to 等。
1.can和could (could为can的过去式) 的基本用法
①表示能力，如：He can speak English better than you.
②在疑问句和否定句中表示“怀疑”、“猜测”或“可能性”，
③表示“许可”时can可以和may换用，
如：You can (may) go home now.
④如果要表示语气婉转，可用could代替can，这时could不再是can的过去式，
如：Could you come again tomorrow?
⑤can和be able to都可表示能力，两者在意思上没有什么区别。但是can只能有现在式和过去式，而be able to则有更多的形式，且can指一般能力，able to 可表示具体能力
如：He will be able to do the work better.
语法易错题
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2、may和might (might为may的过去式)的基本用法
(1)表示允许或征询对方许可，有“可以”之意，
如：You may use my dictionary.
在回答对方说“可以做某事”或“不可以做某事”时，一般多不用may或 may not，以避免语气生硬或不容气。而用比较婉转的说法进行回答。
如：---- May I use this dictionary?
---- Yes, please. 或 ----Certainly.
在请求对方许可时，如果Might I…? 就比用May I…? 语气更婉转些，
如：May I have a look at your new computer?
但是表示“阻止”或“禁止”对方做某事时，要用must not代替may not，
如：---- May we swim in this lake?
---- No, you mustn’t. It’s too dangerous.
(2)may或 might都可以表示可能性，表示“或许”、“可能”之意，如果用might表示可能性，则语气更加不肯定，如：They may (might) be in the library now .
3、must的基本用法
(1)must表示“必须”、“应该”之意，其否定式 must not，缩写形式为 mustn't，表示“不应该”，“不准”、“不许可”或“禁止”之意，
如：We must study hard and make progress every day.
You mustn't touch the fire.
注意：对以must提出的疑问句，如作否定回答时，要用needn't或用don't (doesn't) have to (不 必)来回答，而不用mustn't，因为mustn't表示的是“禁止”或“不许可”之意
如：---- Must we finish the work tomorrow?
---- No, you needn't (don't have to), but you must finish it in three days.
在肯定句中must可以表示推测，表示“一定”或“必定”之意，如：---- Whose new bike can it be?
---- It must be Liu Dong's. I know his father has just bought him a new one.
作业：
1.___ I take the book out?—I'm afraid not.
A. Will B. May C. Must D. Need
2.— May I take this book out of the reading room?
—No, you___ . You read it in here.
A. mightn’t B. won’t C. needn’t D. mustn’t
3 . I have told you the truth. ______ I keep repeating it?
A. Must B.can C. May D. Will
4. -Must I buy a gift?
-You___ buy a gift, but you can if you want to.
A. must B. mustn't C. have to D. don’t have to
5. —I haven’t got the reference book yet, but I’ll have a test on the subject next month.
—Don’t worry. You______ have it by Friday.
A. could B. shall C. must D. may
6. I_______ have watched that movie —it’ll give me horrible dreams.
A. shouldn’t B. needn’t C. couldn’t D. mustn’t
7.--- There were already five people in the car but they managed to take me as well.
--- It ___ a comfortable journey.
A. can’t be B. shouldn’t be C. mustn’t have been D. couldn’t have been
