助动词定义动词表示动作形态，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, shall, will ；▪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.▪构成反意疑问句，如：▪Mr. Chen has not become a businessman, has he?▪Jason's lectures are clear and easy-to-understand, aren't they?▪构成简短答语，如：-Will you help us with the gardening?Yes, I will. /No, I won't.-I have seen that film before.-So have I.-William isn't a teacher.-Neither is his wife.▪构成省略句，如：The boss doesn't often come to the factory and when he does, it isgenerally 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 due to, be going to, be likely to, be bound to, be obliged to, be supposed to, be willing to, have to, seem to, be unable to, beunwilling 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，有时也可与助动词搭配,例如:He is unwilling to help me with my English.James will be obliged to resign.We were about to start when it rained.He seems to be very disappointed.I don’t have to buy a new hou se.You are bound to succeed.情态动词情态动词有can (could), may (might), must, need,, dare (dared), shall (should), will (would), ought to, used to一、情态动词用法1. 表“能力”Can, could , be able toCan 既能表示现在的能力, 也能表示将来的能力; 既能表示具体事情的能力, 也能表示一般的能力.Look! I can do it.I can’t do it now, but I can do it later.I can climb this cliff.He can drive a car.表示将来的能力通常用will/shall be able toI will be able to speak French in two months.Could 所表示的能力泛指过去的一般能力;She could play the piano when she was only six.在肯定句中, 表示过去某桩具体事情的能力, 用was/were able to; 否定句中无此区别.He was able to translate the article without a dictionary.I couldn’t /was not able to play the piano when I was at school.2. 表示“可能”May, might, can, couldMay/might 可交替使用, 无时间上的差异, 表示现在或将来的可能. might语气更加不肯定,更委婉.It may/might be true.He may/might be waiting at the door.He may/might leave tomorrow.疑问句中表示可能, 用can 而不用may;--Where can he be?--He may be in the office.Can /could 表示现在的可能, could更委婉;She couldn’t/can’t love him.Can 较多地用于否定句和疑问句中, could无此限制;Can 表示的可能常指逻辑上的可能性;may则指事实上的可能性; Mr. Reed is in poor health. He can be ill at any time.Mr. Reed looks pale. He may be ill.表示过去的可能He may/might have been hurt.She may/might have missed the train.He can’t/could have been hurt.表示本来可能发生但没有发生You might have finished the work last week.I could have passed the exam, but I failed.3. 表示“许可”Can, could , may, might 无时间上的差异May/might比can/could 正式; might/could 比may/can 委婉; May/might/can/could I smoke here?表示给予许可,用can/may, 而不用could/might;--Could I use your phone?--Yes, of course you can.--Might I trouble you for light?--You may indeed.4. 表示“不许可”May not/ cannot--Can I go out for a moment?-- No, you cannot.--May I use your car for a few days?--No, you may not.表示过去的不许可He was not allowed to go.I di dn’t permit him to go.5. 表示“义务”Should, ought to, mustShould/ought to 带有劝说或敦促的意义, should口气更肯定We should learn from the model workers.You ought to drink less.You should/ought to have asked my permission first.Must 表示敦促或命令, 口气比should更强烈;You must be back by ten o’clock.We must do it now.We will have to do it again.I had to leave at six yesterday.You mustn’t talk like that.--Must you leave so soon?--No, I needn’t.You didn’t need to do it at once.You didn’t have to do it at once.Must/have toHe must say it in English. (I want him to do so)He has to say it in English. (because he doesn’t know Chinese.)6. 表示“必然”Should, ought to, mustShould/ought to 表示必然, 指说话人根据一定情况作出推测或推断;Through the intensive training you should be able to pass the exam without any difficulty.These young trees ought to provide shade in ten years.Must 口气最为肯定.All men must die.Careless reading must give poor results.Her eyes are red. She must have been crying.He can’t be there now.He can’t have been to your home. He doesn’t know the address.7. 表示“预见”Will/shallI will be a different person when I live in England.I shall never do it again.Shall I help you? (征求意见)8. 表示“推测”Should, ought to, must, will/wouldThey must be home by now.(一定)They will be home by now.(估计)They would be home by now.(大概)They should/ought to be home by now. (应该)A lion will attack a man only when hungry.(现在习惯)Before his retirement he would catch the early bus every morning. (过去习惯)9. 表示“意愿” “意图”Will, would, shallWho will go with me?Will you please open the door for me?--Would you pass me the salt, please? (客气的请求)--Certainly, I will. (接受请求)I asked him if he would help me with my writing. (过去的意愿)shall一般适用于第二、三人称主语, 并非主语的意愿,而是说话人的意愿, 意图. You shall stay with us as long as you like.(I’m willing to let you stay as long as …)Shall you take a holiday this summer?10. 表示“决心”Will, would, shall 要重读I will take the job and no one is going to stop me.I won’t go back my words.He would climb the mountain regardless of danger. (过去的决心)You shall obey my orders. (说话人的决心)(I insist that he obey my orders.)11. would/used toWhen we were children, we used to/would go skating every winter.表示过去的习惯, would只可与动态动词连用, used to 既可与动态动词连用, 也可与静态动词连用.可能性排序That must be George. certainwillwouldought toshouldcancouldmaymight uncertain▪dare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。