because,for,since,as的区别：because语气强，表示客观必然原因：Heisabsent,becauseheisill.for语气轻，表示非客观必然的原因，是主观可改变结果的原因，甚至是猜测可能的原因：Heisabsent,forheisbusy.Hemustbeill,forheisabsent.since,as都是不讲自明的原因，是已知的原因：SinceIamaboy,letmecarrythecase.Asyoudon’tfellwell,youhadbetterstayathome.When,while,as引导时间状语的区别：1、When可指时间点，又可指时间段，从句中可用延续性动词，也可用非延续性动词。WhenIgothome,hewashavingsupper.WhenIwasyoung，Ilikeddancing．2、While总是指一个时间段，从句中必须用延续性动词。WhileIslept，athiefbrokein.Don′ttalksoloudwhileothersareworking.3、as表示两个动作同时发生：一个主语同时进行两个动作：一种状态随另一种状态的变化而变化。表示"当……时""一面……一面"，"随着"。从句中多用表示动作的动词，而不用be动词或表示感觉、理解、知道这类动词。Ashestoodthere,hesawtwomenenterthebar.Shedroppedtheglassasshestoodup.Thestudentstooknotesastheylistened.Asitgrewdarkeritbecamecolder.注意：①when从句与主句动作先后发生时，不能与while，as互换。如：Whenhehasfinishedhiswork，hetakesashortrest．(when=after)WhenIgottothecinema，thefilmhadalreadybegun.(when=before)②when从句动词为终止性动词时不能由while替换。如：Whenhecameyesterday，wewereplayingbasketball．说明：此时when也不能用as替换，因为as从句为终止性动词时，主句通常也必须是终止性动词。③当从句的谓语是表动作的延续性动词，when,while和as才有可能互相替代。如：While／When／Aswewerestilllaughing,theteachercamein.正当我们仍在大声嘻笑时，老师进来了。④从句表示"随时间推移"连词能用as，不用when或while。Asthedaywenton,theweathergotworse.日子一天天过去，天气越变越坏。⑤当从句的谓语是终止性动词，而且主句的谓语也是终止性动词时，when可和as通用，而且用as比用when在时间上更为紧凑，有“正当这时”的含义。如：Hecamejustas(或when)Ireachedthedoor.。⑥从句的谓语动词如表示状态时，通常用while。如：Wemuststrikewhiletheironishot.。⑦When,while的其他用法：1.Wewerehavingsupperwhenthetelephonebegantoring.2.Whyusemetalwhenyoucanuseplastic.3.Ilikeplayingfootballwhilehelikestennis.4.WhileIadmithisgoodpoints,Icanseehisshortcoming.5.Whileheloveshisstudents,heisverystrictwiththemwhetherwhether与与ifif均为“是否”的意思。但均为“是否”的意思。但在下列情况下，在下列情况下，whetherwhether不能被不能被ifif所取代：所取代：1.whether引导主语从句（包括从句在句首的位置）应当用whether，不用if，如：Whetheritistrueremainsaproblem.Whetherhewillcome,Iamnotsure.2.引导表语从句用whether，不用if，如：Thequestioniswhetheryoushouldacceptit.Whether;if用法区别3.whether可以引导从句作介词的宾语，而if则不能，如：I’mnotinterestedinwhetherthey’llgoornot.Itdependsonwhetherwehavegotenoughmoney.4.whether可以直接跟动词不定式连用，而if则不能，如：Ididn’tknowwhethertolaughortocry.Shehasn’tdecidedwhethertogoornot.5.引导同位语从句用whether，不用if，如：Thequestionwhetherhe’llattendthemeetingisessential.1.—IthinkGeorgedoesn’treallycareforTVplays.—Right,______hestillwatchestheprogram.(NMET2004重庆)A.andB.butC.orD.so2.______modelingbusinessisbynomeanseasytogetinto,thegoodmodelwillalwaysbeindemand.(NMET2004浙江)A.WhileB.SinceC.AsD.If3.______Iacceptthatheisnotperfect,Idoactuallyliketheperson.(NMET2004江苏)A.WhileB.SinceC.BeforeD.Unless4.Idoeverysinglebitofhousework______myhusbandBobjustdoesthedishesnowandthen.(NMET2004III)A.sinceB.whileC.whenD.as5.______youcallmetosayyou’renotcoming,I’llseeyouatthetheatre.(NMET2004II)A.ThoughB.WhetherC.UntilD.Unless6.Standoverthere______you’llbeabletoseeitbetter.(NMET2004天津)A.orB.whileC.butD.and7.Itwasevening______wereachedthelittletownofWinchester.(NMET2004天津)A.thatB.untilC.sinceD.before8.Wewereswimminginthelake______suddenlythestormstarted.(2004北京春)A.whenB.whileC.untilD.before9.______Icansee,thereisonlyonepossiblewaytokeepawayfromthedanger.(2004北京春)A.AslongasB.AsfarasC.JustasD.Evenif
