Unit3WeshouldobeytherulesLesson18情态动词can、may、must、need解析区别练习情态动词后面+动词原形情态动词没有人称和数的变化Icannotplaybasketball.Ineedbuyacomputer.(能)(需要)Shecanrideahorse.Amyneedbuyabook.否定式构成是在情态动词后面加"not"Icanplaybasketball.(能)用法:①表示能力:HecanspeakalittleEnglish.Canyoufly?③表示请求或允许:CanIhelpyou?②表示猜测:ThatcannotbeourEnglishteacher,becauseshehasgonetoShanghai.could也可表示请求，语气委婉，主要用于疑问句，不可用于肯定句，答语应用canCouldIcometoseeyoutomorrow?Yes,youcan.（否定答语可用No,I'mafraidnot.）用法:①表示请求或允许:MayIgohome，please?(1)以may（表许可或请求）开头的一般疑问句,肯定回答用may，否定回答用mustn't或can't.(2)在疑问句中不用于第二人称②表示可能性:用在肯定句Youmayberight.No,you_________(可以)Yes,youmay.might为may的过去式，但也可以代替may，语气较为婉转客气或更加不肯定。Youmightberight.mustn’t/youcan’t.试比较:●Youmaynotsmokehere．(可能不)语气轻●Youcan’tsmokehere．（不能）重●Youmustn’tsmokehere．（决不可）最重用法:既可以做情态动词,也可以做实意动词②实意动词(needtodosth):①情态动词(needdosth):Ineedbuyabighouse.(需要)Heneeddrivemehome.DoIneedtobuyabighouse?Heneedstobuyabighouse.NeedIbuyabighouse?--Yes,youmust.--No,youneedn’t/don’thaveto.(不必要)--Yes,youdo.--No,youdon’t.用法:②表示推测“肯定，一定”:①表示义务、必要或命令:Youmustcomeearlytomorrow．---MustIfinishmyhomeworkfirst?Theymustbeathome.Thelightison.---Yes,youmust.---No,youdon'thaveto/needn't.(必须)must开头的疑问句，其否定回答通常用don‘thaveto或needn’t而不用mustn't(禁止)③否定时表示“禁止”:Youmustn’tsmokehere.用法:It’seleveno’clock.Ihavetogonow.Itwasgettingdark.Hehadtofinishtheworkfirst.Idon’thavetogonow.Hedoesn’thavetofinishtheworkfirst.(必须)haveto表示客观因素Weareverygladattheparty,butit’s11,sowehavetoleavenow.must表示主观因素Wemuststudyhardtofindagoodjobforourselves.(必须)Mybrotherwasveryill,soIcallthedoctorinthemiddleofthenight.Hesaidthattheyworkhard.Youdon'thavetotellhimaboutit.Youmustn'ttellhimaboutit.你不一定要把此事告诉他。你一...
