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1、情态动词的用法一 can和could情态动词用法例子补充说明can/could表示能力1I can speak fluent English now, but I couldnt last year.Can表现在能力；Could表示过去能力.可用be able to代替；was/were able to 表示成功做了某事在肯定句中，表示客观可能性，并不涉及具体某事会发生，常用来说明人或事物的特征。要表达具体某事实际发生的可能性时，不用can，需用could，may，might。1. I may stay at home this weekend.（实际可能性）2. Peter might co
2、me to join us.(实际可能性) 3. It will be sunny in the daytime ,but it could rain tonight.(实际可能性)1. can用于否定和疑问句(could不限)2. can(be)表示有时候会（常与sometimes,attimes连用）表示请求和允许。表示请求，口语中常用could代替can，使语气更委婉。1 Can we turn the air conditioner on?.2. I wonder if I could just ask you to sign this.1. 请求用could 语气委婉 2. 允许不用
3、 could.表示对现在的动作或状态进行主观的猜测，主要用在否定句和疑问句中。1 It cant be easy caring for a man and a child who are not your own.2 Can the man over there be our head master?表示惊异、怀疑、不相信等态度，主要用在否定句、疑问句和感叹句中。1 Can this be an excuse for not giving them help?2 This cant be true.3 How can you be so crazy!特别提示：（1） could用来表示请求时，
4、语气委婉，主要用于疑问句，不能用于肯定句，回答应该用can（即：could不能用于现在时态的简略答语中）。如：Could I use your dictionary?Yes, you can.(否定回答可用：No, Im afraid not.)(2) can和be able to区分can(could)和be able to都可以表示能力，意思上没有区别。但can只有现在式和过去式，而be able to则有更多的形式。如：Ive always wanted to able to speak fluent English.Those bags look really heavy, are y
5、ou sure youll be able to carry them on your own?但是，表示在过去某时的某一场合经过一番努力，终于做成了某事，通常不用could，而用was/were able to来表示。这时，was/were able to相当于managed to do或succeed in doing。如：After the accident it was a long time before she was able to walk again.The fire was very big, but most people were able to escape from
6、 the building.(3) 惯用形式“cannot too”表示“无论怎么也不（过分）”。如： You cannot be too careful.你越小心越好。 惯用形式“cannot but+ 不定式（不带to）”表示“不得不，只好”。如： I cannot but admire her determination.我不得不钦佩你的决心。 二may和 might情态动词用法例子补充说明may/might表示允许、许可。否定回答一般用must not/mustnt,表示“禁止、阻止”之意，但也可以用had better not (最好别)或may not（不可以），语气较为委婉。1
7、May I come in and wait?2 May I smoke here?No, you mustnt(或No, youd better not.)1. 请求用might语气更委婉。 2. 允许时用may，表示“可以”（表示允许时不用might）。在表示请求、允许时，might比may语气更委婉些。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，在日常用语中，用Can I征询对方意见更为常见。1 Might I borrow your pen?2 I wonder if I might speak to your son.表示可能性的推测，通常用在肯定句和否定句中，含有“
8、或许”“大概”“可能”之意；用might代替may时，则语气显得更加不肯定。1 It may rain this afternoon.2 I suppose he might have missed the train.常用于肯定句（might可能最小）may用于祈使句表示祝愿1 May you succeed！惯用句式：“may well+ 动词原形”，意为“完全能，很可能”，相当于to be very likely to“may as well或might(just)as well+动词原形”意为“最好，满可以，倒不如”，相当于“had better或there is no reason
9、to do anything else.1 There is nothing to do, so I may as well go to bed.2 You may as well tell us now, well find out sooner or later.3 I suppose we might as well go home.二must和have to情态动词用法例子补充说明Musthave to表示“必须，应该”之意，语气比should，ought to强烈。其否定形式mustnt表示“不准，不应该，禁止”等意1 You must come to school on time.
10、2 You mustnt drive so fast in the street.3 We mustnt waste any more time.1. must多表主观、现在/将来义务; have to多表客观、过去义务 2. mustnt表禁止；否定用neednt / dont have to在回答带有must的问句时，否定回答常用neednt或dont have to，表示“不必”，而不用mustnt1Must I come back before ten? Yes，you must.(No, you neednt)表示有把握的推测，意为“一定、准是” 1 It must be my mo
11、ther.2 You must be hungry after a walk.3 There must be a hole in the wall. 只用肯定句。在否定句/疑问句中用can/could“必须，不得不”，意义与must相近。但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。1 The film is not interesting. I really must go now.2 I have to go now, because my mother is in hospital.must只有一种形式，即现在式与过去式都是一种形式，而have to则涉及各种人
12、称、时态等方面的变化形式。1I had to work hard when I was your age.2In order to take the exam, well have to finish the whole book by the end of this month.两者的否定意义不同，mustnt表示“禁止，不许”，dont have to表示不必。1 You mustnt go there.2 You dont have to go there.3四shall和should情态动词用法例句补充说明shall用于第一、二、三人称构成的疑问句，表示征求对方意见或请求指示1 Sha
13、ll I open the window?2 Shall we say 6 oclock, then?3 What shall I get for dinner?表规章、法令、预言：“必须”用于所有人称 Every competitor shall wear a number用于第二、三人称陈述句中，表示说话人给对方的命令、警告、允诺或威胁。1 Dont worry, you shall get the answer this afternoon.(允诺)2 He shall be sorry for it one day, I tell you. （警告）3 You shall do as
14、I say. (命令)4 If you children dont do as I tell you, you shall not go to the party. (威胁) should表示劝告或建议，意为“应该”1 What should I do?2 Should I trust him?3 You should read his new book.表示推测，用在肯定句中，对现在的情况或可能发生的事的主观推测或期待。意为“想必，大概，或许”1 It should be a nice day tomorrow.2 Try phoning Robert, he should be home now.3 He should be around sixty years old.肯定的语气没有must用于推测时强 This pen ought to /should be yours.还可以用在if引导的条件句中，表示一件事听起来可能性很小，但也不是完全不可能，相当于“万一”的意思。从句谓语用should+动词原形构成，主句都一定用虚拟语气1 Ask Tom to ring me up if you should see him. (你万一见到汤姆，请让他给我打个电话)2 Should I be free tomorrow, Ill come. (万一我明天有时间，我就过来)
