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情态动词(Modal verbs)本身有一定的词义，表示语气的单词。情态动词虽然数量不多，但用途广泛，下面小编告诉你八年级英语情态动词的用法，大家一起来看看吧!八年级英语情态动词的用法：一、“情态动词+动词原形”表示对现在的推测。1、can表示推测时一般用于否定句或疑问句。如：That man can"t be her husbandshe is still single.Who is knocking at the door?　Can it be the postman?2、must表示肯定的推测，一般用于肯定句中。如：He must be in his office now.Mr Li must be working now， for the lights in his office are still on.3、might 表示推测时不一定是may的过去时，只是表示其可能性较小。如：The man may be the headmaster.—Where is Mr Li? —He might be working in his office. —May Mr Li come? —He might not come here.4、Could表示推测时，语气can比要弱，说话者留有余地。如：—Could it be an animal?—It could not be， because it is not moving.5、Should表示推测的可能性比较大，仅比must的可能性 小一点。如：It is already 10 o’clock now they should be there.二、情态动词表示对过去可能发生的动作或存在过的动作的推测性用法。1、“must +have done/been------”表示“过去一定发生过某事或存在过某种状态”，不用于“musn’t+have-----”形式。如：She must have seen the film before，hasn’t she?(注意反意疑问句的后半部分)You must have met uncle Wang in the shop yesterday，didn’t you? (注意反意疑问句的后半部分)2、“should +have done /been-------”表示“本应当做某事，而实际上并没有做”;“ shouldn’t+完成式”表示“本来不应当做，而实际上却做了”。以上结构常带有说话者的责备的感情色彩。如：You should have finished your homework earlier (but in fact you did not finish it on time).You shouldn’t have gone to bed when you woke up at five (but in fact you went to bed again then).3、“needn’t+完成式”表示“本来没有必要做某事，而实际上却做了”。如：There was plenty of time. She needn"t have hurried .4、“can’t /couldn’t+have done /been-------”表示“过去不可能发生了某事或存在过某种状态”。如：I saw him just now. He can’t have gone to Japan.She said the man couldn’t have stolen her car.5、“could+have done/been-------”表示“过去本来能够，可以做某事或成为某种状态，而实际上没有”，说话者有些遗憾。“could sb. have done /been-------?”是它的问句形式。如：You could have stayed with the Smiths while you were in New York (but in fact you stayed in a nearby hotel .) Could Mr Li have helped this girl student?6、“may/might+完成式”表示“过去可能，本来可以于某事而实际上没有干”，might的可能性较小，语气较弱。如：He may have finished reading the book. She might have given you some help，however bus she was.情态动词will与would的用法归纳：1. 表示意愿will 表示现在的意愿，would表示过去的意愿。如：I will bring it tomorrow. 我明天会把它带来。We will not go there again. 我们不会再去那里。He asked if I would go to the cinema with him. 他问我是否愿同他一起去看电影。If you will allow me, I will see you home. 如果你同意，我可以送你回家。If you will come, we will be glad. 假如你能来的话，我们就会很高兴。2. 表示征求意见或提出请求will和would均可用，主要用于第二人称的疑问句中，would此时并不表过去，而表示委婉语气。如：Will / would you please tell me the way to the post office? 请告诉去邮局的路好吗?Will / Won’t you have some tea? 你喝点茶，好吗?You wouldn’t have the time to phone him, would you? 你现在没有那么多时间给他打电话是吧?would有时与like, love, hate, prefer, be glad, be happy等连用，用于提出建议或邀请。如：Would you like some cake or biscuit? 你要吃点蛋糕还是饼干?I’d like / love a coffee. 我倒想喝杯咖啡。I would hate you to think I was criticizing you. 我可不愿意让你觉得我是在批评你。3. 表示习惯和倾向性will表示现在，would表示过去。如：A bear will not touch a dead body. 熊通常不碰死尸。Fish will die out of water. 鱼离开水就不能活。This door won’t open. 这扇门打不开。He will sit for hours without saying a word. 他经常一坐数小时而不发一言。She would lose the key! 她总是把钥匙弄丢了。When he was a child, he would often go skiing. 他小时候经常去滑雪。He told me the box wouldn’t open. 他告诉我箱子打不开了。4. 表示推测will用于推测现在，would用于推测过去，也可用于推测现在，语气较委婉。如：This bus will hold 40 people. 这辆大客车可以坐40人。The teacher will be over fifty. 老师的年龄大概过五十了吧?That would be in 1978, I think. 我看这事发生在1978年。That would be her father. 这大概是他妈。有时will / would+完成式，表示对过去可能已经发生的情况的推测。如：He will have heard the news. 他已经听到那消息了吧。I thought you would have finished it by now. 我想你现在可能已经做完了吧。6. 表示执意或决心will表示现在，would表示过去。如：He will have his own way. 他坚持要一意孤行。I won"t have you say such things. 我可不许你说这样的话。He would do it, though I told him not to. 虽然我叫他不要做，可是他偏要做。can’t与may not的用法区别：1. can’t有两个主要用法一是表示“不能”(指没有能力或未得到许可等)，二是表示“不可能”(指没有可能性)。如：You could always hire a dress for the ball if you can’t afford to buy one. 如果你买不起舞会礼服，总可以租一套。(can’t=不能)Grandma always turns the TV up really loud because she can’t hear very well. 奶奶总是把电视声音开得很大，因为她听力不太好。(can’t=不能)That can’t be Mary — she’s in hospital. 那个人不可能是玛丽——她住院了。(can’t=不可能)What you’re saying can’t be true! I can hardly believe it! 你说的话不可能是真的! 我不信!(can’t=不可能)2. may not也有两个主要用法一是表示“不可以”(指不允许)，二是表示“可能不”(指把握不大的否定推测)。如：Books may not be taken out of the library. 图书馆的图书不得携出馆外。(may not=不可以)Students may not bring textbooks into the examination room. 学生不得携带教科书进入考场。(may not=不可以)Don’t go any closer — it may not be safe. 别走近，可能不安全。(may not=可能不)The figures may not be exactly right, but they’re close enough. 那些数字可能并不十分精确，但已十分接近。(may not=可能不)注意：不要误用may not 来表示“不可能”，表示“不可能”要用can’t。needn’t与mustn’t 的用法区别：1. needn’t 的用法needn’t 的意思是“不必”“没有必要”。如：If you can lend me the money I needn’t go to the bank. 如果你能借给我钱，我就不必去银行了。I’m in no hurry. He needn’t send it by air. He can send it by sea. 我不着急。他不必用航空寄来。海运过来就行。2. mustn’t 的用法mustn’t的意思是“不可以”“不允许”。如：He has a weak heart and must not exercise too hard. 他心脏很弱，不能做强烈的运动。Our school has a rule that students must not eat or drink in class. 我们学校规定，学生不许在课上吃喝。请看看下面两组对比性句子：You mustn’t drive fast. There is a speed limit here. 你不能开快车，这地方有速度限制。You needn’t drive fast. We’ve plenty of time. 你不必开得快，我们有足够的时间。You mustn’t strike a match. This room is full of gas. 你不能划火柴，这屋里满是煤气。You needn’t strike a match. I can see well enough. 你不用划亮火柴，我可以看清。