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（一）助动词有be, have, do, will, shall。它们本身没有词义，只和实义动词的一定形式构成复合谓语，用来表示时态和语态，构成否定、疑问及加强语气等。 1、be (am, is, are, were, been) (l)“be + -ing”构成进行时态； （2）“be + 过去分词”构成被动语态； （3）“be + 动词不定式”构成复合谓语： ①表示按计划安排要发生的事。The prime minister is to visit Japan next year. 总理将于明年访问日本。 ② 用于命令。You're to do your homework before you watch TV.你得做完了作业才能看电视。
2、have (has, had) (1)“have+过去分词”构成完成时态。如：Have you seen the film ? (2)“have been + -ing”构成完成进行时态。如：What have you been doing these days? 这些日子你一直在干什么?
3、do (does, did) (1)“do not + 动词原形”构成行为动词的否定式。如：His brother doesn’t like playing basket.； (2)“Do + 主语 + 动词原形”构成行为动词的一般疑问句。Does he go to school by bike every day? (3)“do + 动词原形”用于祈使句或陈述句中表示加强语气。如：I did go to see him, but he wasn’t in我确实去看望他了，但他不在家。Do do some work. 请一定做点什么； (4)代替前面刚出现的动词以避免重复。My mother told me to go to bed early. So I did.
4.will, shall (would, should) “will (shall+动词原形”构成一般将来时，一般来说shall用于第一人称，will用于第二人称或第三人称，口语中常用will代替shll，如：We will have a meeting to discuss the problem.
（二）情态动词 情态动词表示说话人对某一动作或状态的态度，可以表示“可能”、“可以”、“需要”、“必须”或“应当”等之意。情态动词没有人称和数的变化。但不能单独作谓语动词用，必须和不带to的不定式连用构成谓语动词。只有情态动词ought要和带to的动词不定式连用，在句中作谓语用。
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。现将各情态动词的基本用法分述如下：
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”，如：Can this green bike be Liu Dong's? (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? (5)can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，如：He will be able to do the work better.
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. (2)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. (3)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.
4、can, could, may, must后接完成式的用法 (1)can, could后接完成式的用法： ①在否定句或疑问句中表示对过去发生过的事情的“怀疑”或“不肯定”的态度，Could he have said so? ②在肯定句中，可以表示过去可能做到而实际并没做到的事情，有“劝告”或“责备”的语气。 如：---- When did you answer her letter? ---- Only yesterday. ---- It's too late. You could have answered it earlier, I am sure.
(2) may, might后接完成式的用法 ①表示对过去某事的推测，认为某一件事情在过去可能发生了。如果使用might，语气就比较婉转或更加不肯定，如：Mary might have learned some Chinese before. ②可以表示过去本来可以做到而实 际没有做到的事情，有“劝告”或“责备”的语气，如：You didn't do the work well that day. You might have done it better.
(3)must后接完成式的用法：表示对过去某事的推测，认为某事在过去一定做到 了，如：Liu Dong isn’t in the classroom. He must have gone to the library.
5、have to 的基本用法：have to和must的意义相近，只是 must侧重表示说话人的主观看法，而have to 则表示客观需要，如：I must study hard. I had to give it up because of illness.
6、ought to的基本用法 (l)表示根据某种义务或必要“应当”做某事，语气比should强，例如：Everyone ought to obey the traffic regulations. (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) ； (3)“ought + have+ 过去分词”表示过去应做某事而 实际未做。例如：You ought to have helped him. (but you didn’t) 这时，ought与 should可以互相换用。注意，在美国英语中ought to 用于否定和疑问句时to可以省略。例如：Ought you smoke so much? You oughtn’t smoke so much. 7、dare的基本用法 (l)dare (dared为其过去式) 作情态动词用时，主要用于否定句，疑问句和条件从句中，如：Dared he bread the traffic regulations again? (2)在现代英语中dare常用作行为动词，其变化与一般行为动词相同，如：She dares to stay at home alone at night.
8、need的基本用法 (1)need作情态动词用时，主要用于否定句和疑问句中，如：He needn't worry about us now. (2)need也可作为行为动词用，可用于肯定句，否定句和疑问句中，其后可接名词、代词、动名词或带 to 的动词不定式为其宾语。如：You need to practise reading aloud every day. (3)needn't后接完成式可以表示过去做了一件本来不必要做的事情，如：---- Did you answer the letter yesterday? ---- Yes, I did. ---- But you needn’t have answered it.
9、shall的基本用法 (1)shall用作情态动词时，用于第二、三人称，表示说活人的意愿，可表示“命令”、“警告”、“强制”、“威胁”或“允诺”等意，如：He shall go first, whether he wants to or not. (2)在疑问句中，shall用于第一、三人称，表示说话人的征询对方意见或请求指示，如：Shall I open the door?
10、should的基本用法 (1)should作为情态动词可以表示“建议”或“劝告”，有“应该”之意，如：You should learn from each other. (2)should后接完成式表示过去没有做到本来应该做的事情，或是做了本来不应该做的事情。如：You should have give him more help.
11、will的基本用法 (1)用于各人称，可以表示“意志”或“决心”，如：I have told him again and again to stop smoking, but he will not listen. (2)在疑问句中用于第二人称，表示说话人向对方提出请求或询问对方的意愿，如：Will you please tell me how to get to the Capital Gymnasium? (3)will可以表示一种习惯性的动作，有“总是”或“会要”之意，如：Every morning he will have a walk along this river.
12、would的基本用法 (1)would作为will的过去式，可用于各人称，表示过去时间的“意志”或“决心”，如：He promised he would never smoke again. (2)在疑问句中，用于第二人称，表示说话人向对方提出请求或许问对方的意愿时，比用will的气更加婉转，如：Would you like some more coffee? (3)在日常生活中，学用“I would like to…”表示“我想要”或“我愿意”之意，以使语气婉转，如：I would like to do Ex.2 first. (4)would可以表示过去的习惯动作，比used to正式，并没有“现已无此习惯”的含义。如：Last year our English teacher would sometimes tell us stories in English after class. / During the vacation he would visit me every week. (5)表料想或猜想，如：It would be about ten when he left home./ What would she be doing there?/ I thought he would have told you about it.
13、used to, had better, would rather的用法 (1)used to表示过去的习惯动作或状态，现在已不存在，在间接引语中，其形式可不变，例如：He told us he used to play foot ball 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/ Used you to go to the same school as your brother? 否定句：I usedn’t to / didn’t use to go there. (usedn’t 也可写作usen’t)；否定疑问句：Usen’t you to/ Didn’t you use to be interested in the theatre? 强调句：I certainly used to/ did use to smoke, but it was a long time ago.；其反意疑问句或简略回答中，也有两种形式：She used to be very fat. didn’t she?/ use(d)n’t she? Did you use to play chess? Yes, I did./ Used you to get up early in the morning? Yes, I did./ used to. (2)had better意为“最好”，后接不带to的不定式，例如：---- We had better go now . ---- Yes, we had (we'd better/ we had better)./ Hadn’t we better stop now? (Had we better not stop now?)/ I think I’d better be going. (用于进行时态，表“最好立即”)/ You had better have done that (用于完成时态，表未完成动作) 注：had better用于同辈或小辈，对长辈不可用。 (3)would rather意为“宁愿”，表选择，后接不带to的不定式，例如：I’d rather not say anything./ Would you rather work on a farm?/ ---- Wouldn’t you rather stay here? ---- No, I would not. I’d rather go there. 由于would rather表选择，因 而后可接than，例如：I would rather work on a farm than in a factory. / I would rather watch TV than go to see the film/ I would rather lose a dozen cherry trees than that you should tell me one lie./ I’d rather you didn’t talk about this to anyone. (句中的 'd rather不是情态动词，would 在此是表愿望的实义动词) |
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