主语从句详解一、概述在句子中起名词作用的多种从句统称为名词性从句.根据它们在句中的语法作用, 此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、主语从句重要有四类（1） 由连词th引导的主语从句:引导词tht无含义/在句中不做成分/不可以省.例如:That you ill win the medal semskly.That you aoiniffeent boters e.Thtsh survied the accden isa miace.(2）用连词 whetr 引导的主语从句： whethr有含义(与否）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：Whethr wll hold a prty nthe openair tomorow depend o the ather hthrshe scomin r no dosntatt too muc.（3) 用连接代词引导的主语从句在由连接代词who, hoe, hom,which, wat, whoer,whmeve,wicev, atevr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:Wat oneed isore rtice. hat nt to kw is thi.Wateve we ds toserve h peple.注:watever / heve的功用whtevr,wevr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whatr = anin ht; whoeer = ayn who.要注意和whatever，wver引导的让步状语从句的区别.Whov reks teaw shold beunised.(主语从句)(ne wo reak the law ould e unised. )Whovr braks the l, shoul e punished. (让步状语从句）(N matter o breas the law,e shoud be unishe. ）（4）用连接副词when, here, wy, how引导名词性从句（其连接副词有含义,在句中作状语.)例如:ere e shoul eveit is a prom.en hey ill cme han emde pubic.三、注意点: it构成的主语从句()由连词tha引导的主语从句, 在多数状况下放到句子的背面， 而用代词it作形式主语.例如:That lgh trvls in sraiht ine is knw to al.=It isnown all h liht travels in taight lns.Whenhe plae s toake ffhs ot bee auced . I has no benannnc when te lne is t takeof(但当wat引导的主语从句表达“的东西”时, 一般不用it作形式主语.)错:t is boo wht h wants.对：Wha h wats is bok. 此外, 需要注意的是, it作形式主语替代主语从句时， 要注意和as引导的定语从句的区别.试比较: was rpotdthat the US was under thetrroit atack. As was repotd, the U as dr therroist tt. (2)常用用it作形式主语的复合句构造A I is +名词+从句t is a actha事实是I is goo new tht是好消息It sa quetin that 是个问题It s comn owlede hat 是常识类似的名词尚有：a iy；a won；a goodthn；nwond;urpris等例如:I ismstry toe hwit all hppeneIt s common nowleget the whal isot fi.t is no surpriethat Bob should hav won the gameIts pit tat u missed te ilm. Ii +形容词+从句Its ecessryhat 有必要It iclerthat 很清晰It is liely tht 很也许It is iportat tht 重要的是类似的形容词尚有：Strnge; atual; obis;tre； good;woerful; ossible; likely； que; unusual； certan; evident；wor-whie;urpriig； inersig; astonisn, ec例如：I sdoutul whehr she wi be ble tocome.t s all stonshi hat he refsed o tak toyu.It is essetal th he sholbe her y the eeknd.t seem obviuthat w cannot go lik his.It is necsay tat you （shoul）master the computer.t s importatht a tuent larn Engish wel. It is likely tat a hurran wilare oo.需要注意的是, 此类主语从句中, 谓语动词诸多为“（shold）+动词原形”,即要用虚拟语调. is +过去分词+从句t is sadthat 据说It seprted that 据报道t ha ben prve that 已证明Itmuste proved that必须指出类似的过去分词尚有：kwn; stimated；xpected;belived; thouht； he；noed;iscussed;eqied; eidd; sugeed; emande;mde clar； nd o，例如: is toght hateis te best layr.I is estimatd thate vass ears olIt s use t b thought tha a ewstrtbe u t colin beeen wo tars. It hasot been mad clearwhen newroad i to epnd totaffiIt is ad hat he wasiled n theearthuke. D.Itsems（apene/ appers/ dosn attr/makes o dfeenc/ ocrd )thaIt ses t hey will win the gam. t make oiffenc wetherhe will en he meeng r ot. Idoesnot mattr if I msedy trainIt aedtat I saw himyesterday.主从练习1._smuchne-fftholltimbharvestedsntsedA.Testimate .TestmatedCeyreestiate Dtisestmatedthat2._soaals caeolveintheeasnoknown.AWichSie.AthougD.How3._wehaveachedsatribudtoteguidaneofourparent.WhvehateverCHoweveD.Tht._welhydoesotnecsslymeathatamanisgreedy.AFrreasoththesB.JtbcaehesCTherenbeingD.Ttheis5.Alhough_henedinhidvelopedcotrysounliesciecefctn，itcouldocur elsewheeintherld.A.hiB.hat.howD.it6._hehadfgottentotkehotebk.A.ThaccuredtoherSheoccredthatCTohrttoccurredD.Itourredthertht7.asetht_shesaidswrong.AwhichB.C.this.wat8.Weostouayin