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1、名词性从句之主语从句一、名词性从句概述：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、主语从句定义：在句中起主语作用的从句称为主语从句。引导主语从句的连接词主要有：单纯连词（that,whether/if）、连接代词(what,whatever,who,whoever,whom,whose,which,whichever等)和连接副词(when,where,why,how,whenever,wherever,however等)。1)
2、that引导的主语从句（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.你能得奖看起来不大可能。Thatlighttravelsinstraightlinesisknowntoall.众所周知，光沿直线传播。Thatyoudon’tlikehimisnoneofmybusiness.你不喜欢他不管我的事。（2）形式主语it替代主语从句位于句首，that引导真正的主语从句会放到句子的后面。常用的句式主要有如下几种
3、：1.It+系动词+形容词+that从句。如：Itisnecessarythat…有必要……Itisclearthat…很清楚……Itislikelythat…很可能……Itisimportantthat…重要的是……类似的形容词还有：strange;natural;obvious;true;good;wonderful;possible;unlikely;quite;unusual;certain;evident;worth-while;surprising;interesting;astonishing,etc.例如
4、：Itwasreallyastonishingthatherefusedtotalktoyou.Itisessentialthatheshouldbeherebytheweekend.Itseemsobviousthatwecannotgoonlikethis.Itisnecessarythatyou（should）masterthecomputer.ItisimportantthatastudentlearnEnglishwell.It’sclearthattheybadlyneedhelp.Itislikelytha
5、tahurricanewillarrivesoon.知识拓展：在“Itisnecessary/important/surprising/strange/unthinkable/unbelievable/incredible+that从句”这类主语从句结构中，从句中谓语动词常用“（should）＋动词原形”，形式，即要用虚拟语气,should可省略。例如：Itisnecessarythatacollegestudent(should)masteroneortwoforeignlanguages.大学生掌握一两门外语是有必要
6、的。Itisstrangethatyou(should)trustJane.很奇怪你竟然相信简。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构2.It+系动词+名词+that从句如：Itisafactthat…事实是……Itisgoodnewsthat
7、………是好消息Itisaquestionthat………是个问题Itiscommonknowledgethat………是常识类似的名词还有：apity；awonder；agoodthing；nowonder；surprise等。例如：Itisamysterytomehowitallhappened.Itiscommonknowledgethatthewhaleisnotafish.ItisnosurprisethatBobshouldhavewonthegame.It’sapitythatyoumissedthefilm.3
8、.It+be+动词的过去分词+that从句如:Itissaidthat…据说……Itisreportedthat…据报道……Ithasbeenprovedthat…已证明……Itmustbeprovedthat…必须指出……类似的过去分词还有：known;estimated;expected;believed;tho
