情态动词有can(could),may(might),must,haveto,shall(should,will(would),dare(dared),need(needed),oughtto等.情态动词无人称和数的变化；不能单独使用,必须与其后的动词原形构成谓语一、can,could1)表示能力（体力、知识、技能）.Canyouliftthisheavybox?（体力）Marycanspeakthreelanguages.（知识）Canyouskate?（技能）此时可用beableto代替.Can只有一般现在时和一般过去式；而beableto则有更多的时态.I’llnotbeabletocomethisafternoon.当表示“经过努力才得以做成功某事”时应用beableto,不能用Can.如：Hewasabletogotothepartyyesterdayeveninginspiteoftheheavyrain.2)表示请求和允许.-----CanIgonow?-----Yes,youcan./No,youcan’t.此时可与may互换.在疑问句中还可用could,might代替,不是过去式,只是语气更委婉,不能用于肯定句和答语中.----CouldIcometoseeyoutomorrow?----Yes,youcan.(No,I’mafraidnot.)3)表示客观可能性（客观原因形成的能力）.They’vechangedthetimetable,sowecangobybusinstead.Thishallcanhold500peopleatleast.4)表示推测（惊讶、怀疑、不相信的态度）,用于疑问句、否定句和感叹句中.Canthisbetrue?Thiscan’tbedonebyhim.Howcanthisbetrue?二、may,might1)表示请求和允许.might比may语气更委婉,而不是过去式.否定回答时可用can’t或mustn’t,表示“不可以,禁止”.----Might/MayIsmokeinthisroom?----No,youmustn’t.----May/MightItakethisbookoutoftheroom?----Yes,youcan.(No,youcan’t/mustn’t.)用MayI...?征徇对方许可时比较正式和客气,而用CanI...?在口语中更常见.2）用于祈使句,表示祝愿.Mayyousucceed!3)表示推测、可能性（不用于疑问句）.might不是过去式,它所表示的可能性比may小.1．Hemay/mightbeverybusynow.2．Yourmothermay/mightnotknowthetruth.三、must,haveto1)表示必须、必要.Youmustcomeintime.在回答引出的问句时,如果是否定的,不能用mustn’t（禁止,不准）,而用needn’t,don’thaveto（不必）.----Mustwehandinourexercisebookstoday?----Yes,youmust.----No,youdon’thaveto/youneedn’t.2)must是说话人的主观看法,而haveto则强调客观需要.Must只有一般现在时,haveto有更多的时态形式.1．heplayisn’tinteresting,Ireallymustgonow.2．IhadtoworkwhenIwasyourage.3)表示推测、可能性（只用于肯定的陈述句）1．You’reTom’sgoodfriend,soyoumustknowwhathelikesbest.2．Yourmothermustbewaitingforyounow.四、dare,need1)dare作情态动词用时,常用于疑问句、否定句和条件从句中,过去式形式为dared.1．HowdareyousayI’munfair?2．Hedaren’tspeakEnglishbeforesuchacrowd,darehe?3．Ifwedarednotgotherethatday,wecouldn’tgetthebeautifulflowers.2)need作情态动词用时,常用于疑问句、否定句.在肯定句中一般用must,haveto,oughtto,should代替.1．Youneedn’tcomesoearly.2.----NeedIfinishtheworktoday?----Yes,youmust./No,youneedn’t.3)dare和need作实义动词用时,有人称、时态和数的变化.在肯定句中,dare后面常接带to的不定式.在疑问句和否定句中,dare后面可接带to或不带to的不定式.而need后面只能接带to的不定式.1.Idaretoswimacrossthisriver.2.Hedoesn’tdare(to)answer.3.Heneedstofinishhishomeworktoday.五、shall,should1)shall用于第一人称,征求对方的意见.Whatshallwedothisevening?2)shall用于第二、三人称,表示说话人给对方的命令、警告、允诺或威胁.1.Youshallfailifyoudon’tworkhard.(警告)2.HeshallhavethebookwhenIfinishit.(允诺)3.Heshallbepunished.(威胁)六、will,would1)表示请求、建议等,would更委婉.Will/Wouldyoupassmetheball,please?2)表示意志、愿望和决心.1.Iwillneverdothatagain.2.Theyaskedhimifhewouldgoabroad.3)would表示过去反复发生的动作或某种倾向.would表示过去习惯时比usedto正式,且没有“现已无此习惯”的含义.1.Duringthevacation,hewouldvisitmeeveryotherday.2.Thewoundwouldnotheal.4)表示估计和猜想.Itwouldbeaboutteno’clockwhenshelefthome.七、should,oughtto1)should,oughtto表示“应该”,oughtto表示义务或责任,比should语气重.1.Ishouldhelpherbecausesheisintrouble.2.Yououghttotakecareofthebaby.2)表示劝告、建议和命令.should,oughtto可通用,但在疑问句中常用should.1.Youshould/oughttogotoclassrightaway.2.ShouldIopenthewindow?3)表示推测should,oughtto(客观推测),must(主观推测).1.Hemustbehomebynow.(断定他已到家)2.Heoughtto/shouldbehomebynow.(不太肯定)3.Thisiswheretheoilmustbe.(直爽)