主语从句详解一、概述在句子中起名词作用的各种从句统称为名词性从句.根据它们在句中的语法作用,这类从句又可分为主语从句、同位语从句、表语从句和宾语从句.二、主语从句主要有四类（1）由连词that引导的主语从句:引导词that无含义/在句中不做成分/不可以省.例如：Thatyouwillwinthemedalseemsunlikely.Thatyouaresoindifferentbothersme.Thatshesurvivedtheaccidentisamiracle.（2）用连词whether引导的主语从句:whether有含义(是否)/在句中不做成分/不可以省.注意：引导主语从句,不能用if,只能用whether.例如：Whetherwewillholdapartyintheopenairtomorrowdependsontheweather.Whethersheiscomingornotdoesn’tmattertoomuch.（3）用连接代词引导的主语从句在由连接代词who,whose,whom,which,what,whoever,whomever,whichever,whatever引导的名词性从句中,其连接代词在句中起名词性作用,担当一定成分.例如：Whatyouneedismorepractice.WhatIwanttoknowisthis.Whateverwedoistoservethepeople.注：whatever/whoever的功用whatever,whoever在主语从句中不含疑问意义.它引导主语从句,并在从句中作主语、宾语、表语等.whatever=anythingthat;whoever=anyonewho.要注意和whatever,whoever引导的让步状语从句的区别.Whoeverbreaksthelawshouldbepunished.（主语从句）（＝Anyonewhobreaksthelawshouldbepunished.）Whoeverbreaksthelaw,heshouldbepunished.（让步状语从句）（＝Nomatterwhobreaksthelaw,heshouldbepunished.）（4）用连接副词when,where,why,how引导名词性从句(其连接副词有含义,在句中作状语.)例如:Whereweshouldleaveitisaproblem.Whentheywillcomehasn’tbeenmadepubic.三、注意点:it构成的主语从句（1）由连词that引导的主语从句,在多数情况下放到句子的后面,而用代词it作形式主语.例如：Thatlighttravelsinstraightlinesisknowntoall.=Itisknowntoallthatlighttravelsinstraightlines.Whentheplaneistotakeoffhasnotbeenannounced.=Ithasnotbeenannouncedwhentheplaneistotakeoff.(但当what引导的主语从句表示“…的东西”时,一般不用it作形式主语.)错:Itisabookwhathewants.对:Whathewantsisabook.另外,需要注意的是,it作形式主语代替主语从句时,要注意和as引导的定语从句的区别.试比较：ItwasreportedthattheUSwasundertheterroristattack.Aswasreported,theUSwasundertheterroristattack.（2）常见用it作形式主语的复合句结...