语法专项训练之情态动词情态动词（modal verbs）：无人称和数的变化，只能与行为动词或状态动词构成谓语既有情态助动词特征又具有实义动词特征的称为半情态助动词情态助动词：can(could),may(might),must,have to(had to),ought to,shall(should),will(would);半情态助动词:dare ,need,used to,had better,would rather;(1)情态助动词1、can 的用法1）表示能力(ability)，意为“能够”Can you finish the work tonight?2)表示请求或允许（permission）,意为“可否” ， “可以”Can I smoke here?3)*表示可能性（possibility） ，常用在否定句，[疑问句或感叹句（含惊讶、怀疑、不相信的语气） ，]意为“可能”Jack cant’t have arrived yet, otherwise he would have telephoned me.Can it be true?How can you be so careless!4)can be 用于推测某人或某物一时的情况，意为“有时候会……”Drinks in restaurants can be very expensive.5)惯用语cannot……too 意为“无论怎样……也不为过；越……越好”I cannot thank you too much.You cannot be too careful in doing exercises. cannot help doing 意为“禁不住”2、may 和 might 的用法1）表示请求允许(即请求别人允许自己做某事)May I sit here?2）表示推测 两者都可用，只是 might 比 may 语气更不确定，表示的可能性更小。
He may know the answer.(可能性较大)He might know the answer.（可能性较小）3）might 常用于表示委婉的请求或者轻微的责备You might have let me known before!4)惯用语may/might (just)as well 意为“最好” （= ）,用于提出建议 You may as well give him the letter.May/might as well +do A + as +do B 意为“与其做 B 不如做 A”You might as well throw the money as lend it to him.3、must 和 have to 的用法1)*must 表示有把握的推测：（否定：cannot be）①对现在情况的推测：must +动词原形The lecture hall is so crowded. There must be over 600 people in it.②表示对过去事情的推测: must+完成时You must have left your umbrella in the theatre.你一定把雨伞丢在剧院里了。
His car is still here. He must have gone by bus.他的车还在这里，他一定乘公共汽车走了③对现在正在进行中的事的推测：must be +现在分词You look happy .You must be having a good time.2)have to 表示“必须” ， “不得不” ，在这个意义上与 must 很接近，但 must 表示的是说话人的主观看法，而 have to 表示的却是客观需要I have to go now.我得走了 （客观需要）I must go now.我必须走 （主观看法）You have to work hard to make a living.为了谋生你就得努力工作 （客观需要）You must do what I tell you.你必须按照我告诉你的去做 （主观要求）（5）have to 有更多的时态形式：现在时，过去时，将来时等，而 must 只有现在时形式We had to be there at 8.我们得 8 点到那里I shall have to go to school tomorrow.明天我得去上学了。
6）have to 不能代替表推测的 must，但在虚拟条件句中用 had had to+动词原形表示与过去相反的情况You must be joking.你一定是在开玩笑 （不能用 have to）If I had had to do the work, I should have done it in a different way.如果我不得不做这项工作，我会以不同的方式去做 （不能用 must）(二)半情态动词既有情态助动词特征又具有实义动词特征情态动词 实义动词肯定式 He dares to escape.He needs to escape.否定式 He daren’t escape.He needn’t escape.He doesn’dare(to) escape.He doesn’t need (to)escape.疑问式 Dare he escape?Need he escape?Does he dare to escape?Does he need to escape?(3)情态助动词+不定式的完成式结构 含义 例句1、could have done本可以，差点就要The accident could have been prevented.2、must have done一定已做 He must have been to shanghai.3、may/might have done或许已经，可能做He may have finished the exercises.4、should have done本来应该做而实际没做“本应该做”You should have started earlier,but you didn’ t.5、needn’t have done本来不必做而实际上已经做了“本不必做”You needn’t have watered the flowers,for it is going to rain.(四)总结事实上的may must可能性 必然性 can have toYou may lead a horse to the water,but you cannot make it drink.说话人为权威may must允许 责任义务can have to （五）练习 权威方非说话人（客观规定）1.They ___have arrived at lunchtime but their fight was delayed. IA. will B. can C. must D. Should2.No one ___be compared with Yao Ming in playing basketball. A. can B. need C. must D. Might3.—I left my handbag on the train, but luckily someone gave it to a railway official. —How unbelievable to get it back! I mean, someone ___it.A. will have stolen B. might have stolen C. should have stolen D. must have stolen4.It ___be the postman at the door. It’s only six o’clock. A. mustn’t B. can’t C. won’t D. needn’t5.If you ___go, at least wait until the storm is over. A. can B. may C. must D. Will6. I ___worry about my weekend—I always have my plans ready before it comes. A. can’t B. mustn’t C. daren’t D. needn’t7. The police still haven’t found the lost child, but they’re doing all they ___. A. can B. may C. must D. Should8.You ______ buy a gift, but you can if you want to. A. must B. mustn't C. have to D. don't have to9.Just be patient .You ______ expect the world to change so soon . A. can’t B. needn’t C. may not D. whether10.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。