助动词

助动词Auxiliaries动词表示动作形态，但英语里有些动词并不表示动作形态，而是协助主动词表示时态（tenses）或语态（voices）。

这些动词叫助动词（auxiliary verbs）英语中助动词主要有三类：一是基本助动词，如have, do, be；二是半助动词, 如be about to等；三是情态助动词，如may, must, need等。

基本助动词助动词可表示动词时态和语态,构成疑问句和否定句,例如“have, has, had”用来构成完成时态；“be, is, are, am, been, was, were, being” 构成被动语态What do you do every day?Betty has completed her work on time. Jason is writing a new book. A rat was caught by a cat.构成疑问句附加语，如：Mr. Chen has not become a businessman, has he?Jason's lectures are clear and easy-to-understand, aren't they?构成省略句，如：The boss doesn't often come to the factory and when he does, it is generally on Sunday.半助动词在功能上介乎主动词和助动词之间的一类结构，称为半助动词。

常见的半助动词有be about to, be apt to, be bound to, be due to, be going to, be liable to, be likely to, be obliged to, be supposed to, be willing to, have to, seem to, chance to, have got to, turn out to, tend to等。

它们可与主动词搭配构成复杂动词词组并表示情态意义，从而像是情态助动词；有时也可与助动词搭配,像是主动词，例如: I have to buy a new car.He seems to be disappointed.I don’t have to buy a new car.He doesn’t seem to be disppointed.情态动词情态动词有can (could), may (might), dare (dared), shall (should), will (would) , must, need, ought to, used to.一、情态动词+动词原形Can1).can, could 表示能力①表示现在的能力,用can或be able toAs I hve got enough money, I can/am able to help her.②表示将来的能力，可用can, 但更常用will/shall be able toI’ll be able to speak German in another two months.③表示过去的能力，在肯定句中，could泛指过去的一般能力,做某件具体事情的能力常用was/were able to，即成功地做成了某件具体的事。

She could play the piano when she was only six.He was able to flee Europe before the war broke out.= He managed to flee Europe before the war broke out.但以上区别仅就肯定句而言，在否定句中无此限制。

2）表示客观可能性和推测Man cannot live without air. Can it be Tom?3)Can/could表示请求和允许Can I go now? Yes, you can. Could I come to see you tomorrow?Yes,you can. No,I am afraid not.4)表示惊异，怀疑，不相信（主要用于否定，疑问或感叹句）This can not be done by him.How can you be so careless!Can this be true?5) can not help but doI can not help but wonder what he is doing.6). can not help doingcan not help (to) do7). can not (never, hardly) do …too… 越．．就越好；无论做．．都不过分a. You can not be too careful in the exam.b. You can not praise him too highly.Maymay表示“允许，可以”，相当于be allowed toYou may go now.May I use your computer?may或might可和as well连用，表示“建议”，译为“还是……的为好”You may as well keep a certain distance from that mad man.You might as well go home now.1）表示允许或请求当表示请求，允许时，might比may语气更委婉，否定答语要用must not,表示“不许可”、“不应该”、“不行”,或can not。

e.g. May I come in?----Yes, you may\can.----No, you mustn‟t\can‟t.2).表示没有把握的推测. “也许”(用于肯定句或否定句中,不用于疑问句)注意：might 表示推测时，不表示时态, 只是可能性比may 小。

e.g. a. He may be playing football.b. He might be at home。

3).may 放在句首，表示祝愿e.g. a. May God bless you! b. May you succeed! = Wish you success! Mustmust表示“必须”或“应当”、“一定”在回答must引起的问题时，如果是否定的答复，不能用mustn‟t，而要用needn‟t 或don‟t have to，因为mustn‟t是“一定不要”、“一定不能”的意思。

You must come to the classroom before eight.“Must we do it now?” “No, you needn‟t.”Must vs. have to1) have to 加动词原形，表示“不得不”，“必须”，表示客观的需要，而must表示说话人主观上的看法，即主观上的必要My brother was very ill, so I had to call the doctor in the middle of the night. (客观上需要做这件事)b. He said that they must work hard. （主观上要做）2) have to可用于多种时态，而must一般用于现在时，其过去时与将来时分别由had to与shall / will have to3）在否定结构中：don‟t have to表示“不必” mustn‟t表示“禁止”e.g. a. You don…t have to tell him about it. b. You mustn…t tell him about it.4)must用于疑问句中，表责备，抱怨，常译为“偏要，硬要，干嘛”Tom, look at the time, must you play the piano at such a late hour?Shouldshould 表示“劝告”，“建议”或“义务”时，译作“应当”，或表示“预测”和“可能”He should take care of his parents as they are old enough not to live on themselves.He should be there now.should have done在虚拟语气中表示“责备或后悔ought，只有一种形式，即ought后必须加to，然后跟接动词原形表示“有义务”或“必要”做某事，译为“应当，应该”You ought not to make fun of him. He is not the one you laugh at but learn from.1)用于第一人称疑问句中询问对方的意愿，但语气较委婉温和What should we do now? 我们现在该怎么办？2)表示应该、必须，常与must 换用。

We should (must) master a foreign language at least.3）表示推断，意思是“很可能，按理应该，估计”It is Monday, he should be at his office.4) should have done 表示过去应该做而实际没有做should not have done 表示过去不该做而实际做了．5)表示惊异，意外等情绪，常译为“竟然”It seems unfair that this should happen to me.6)用于条件状语从句中“万一”If it should rain tomorr ow, we wouldn‟t go camping.Daredare作为情态动词时，主要用于疑问句和否定句，表示“敢”，后面跟不带to的动词不定式。

如：How dare she do things like that to me? Dare you catch the mouse?Dare也用作实义动词，其后的动词不定式可带to也可不带to，且dare有人称和数以及时态的变化。

例如：I dare to jump down from the top of the wall.She doesn't dare (to) meet her teacher's eyesThe little girl ____ the forest.A. didn‟t dare go toB. dared not go toC. dared not to go toD. dares not go toNeedneed意为“需要”，既可作情态动词，也可作实义动词.作情态动词无人称或数的变化，后接动词原形，只能用于否定句和疑问句，如：You needn't worry.Need you go before ten o‟clock?need 作为行为动词有人称和数的变化，后面可接名词、代词、动名词及带to 的动词不定式；可用于肯定句、疑问句和否定句中，如：She needs help.I don‟t need to see the doctor.Do you need to go at once?Need为行为动词时：1) need+sth/to do 2) 表被动sth+need doing/to be done Willwill (would)决心,愿望。