名词性从句名词性从句相当于名词，可分别作主句的主语、表语、宾语和同位语。
因此，名词性从句可分为主语从句、表语从句、宾语从句和同位从句。
引导词：主语从句就是在复合句中用作主语的从句，引导主语从句的引导词主要有三类：一是that；二是whether；三是那些可以用作疑问词的词语，如what, who, which, when, where, how, why等（一）引导名词性从句的连接词1、连接代词：who, whose, whom, what, which。
有词义，在从句中担任成分，如主语、表语、宾语、或定语等。
2、连接副词：when, where, why, how。
有词义，在从句中担任成分，作状语。
3、连接词：that, whether, if, as if。
that 无词义，在从句中不担任成分，有时可省略；if (whether), as if虽有词义，但在从句中不担任成分。
注意：连接代词与连接副词在句中不再是疑问句，因而从句中谓语不用疑问式。
连接代词与连接副词在从句充当句子成分，连接词whether 和if（是否），as if（好象）在从句中不充当句子成分，只起连接作用。
根据句义，如果连接代词与连接副词，whether、if 和as if都用不上时，才用that作连接词（that本身无任何含义）。
一、主语从句与宾词从句连词、语序、时态相同。
只是不能用if, that 不可省；（一）、主语从句是一个句子，在句子中作主语成分。
（二），主语从句的特点1.与宾语从句使用同样的连接词，只是if 不能用；1）、陈述句用that。
2）、一般疑问句用whether。
3）、特殊疑问句用特殊疑问词what, which, who,when, where, why, how 等词引导。
2.经常隐身（以it为形式主语出现）。
例句：1.It's true that the earth is round.That the earth is round is true.2. 表是否的意思时，不能用if代替whether.It hasn't been decided whether he'll come or not.Whether he'll come or not hasn't been decided.3. 从句语序为主谓宾正常语序。
Why didn't he come?Why he didn't come is not known.练习：1.________ makes mistakes must correct them.A. WhatB. ThatC. WhoeverD. Whatever2. It worried her a bit ____ her hair was turning gray.A. whileB. thatC. ifD. for3.When and why he came here ________ yet.A. is not knownB. are not knownC. has not knownD. have not known4. ________ Tom liked to eat was different from ________.A. That…that you had expectedB. What …that you had expectedC. That…what you had expec tedD. What…what you had expected5.________ we go swimming every day ________ us a lot of good.A. If...doB. That...doC. If...doesD. That...does6.It ________ Bob drives badly.A. thinks thatB. is thought whatC. thought thatD. is thought that7.It's uncertain ________ the experiment is worth doing.A. ifB. thatC. whetherD. how8.________ the boy didn't take medicine made his mother angry.A. ThatB. WhatC. HowD. Which9.____ you don't like him is none of my business.A. WhatB. WhoC. ThatD. Whether10.____ we'll go camping tomorrow depends on the weather.A. IfB. WhetherC. ThatD. Where11.______he won't go there is clear to all of us.A. HowB. WhatC. WhyD. This12._______you come or not is up to you.A. WhatB. IfC. WhyD. Whether13._______team will win the match is a matter of public concern.A. WhichB. ThatC. IfD. How14.________leaves the room last ought to turn off the lights.A. AnyoneB. The personC. WhoeverD. Who二、同位语从句（一）、含义1、在主从复合句中作同位语的从句称为同位语从句；所用连词与宾语从句（除不能用if）和主语从句相同2、连词1）、陈述句用that。
2）、一般疑问句用whether。
3）、特殊疑问句用特殊疑问词what, which, who,when, where, why, how 等词引导。
3、常放在fact, news, idea, truth, hope, problem, order, information, wish, promise, answer, evidence, report, explanation, suggestion, conclusion,等抽象名词后面，说明该名词的具体内容。
换言之，同位语从句和所修饰的名词在内容上为同一关系，对其内容作进一步说明。
例：The news that they had won the game soon spread over the whole school.他们比赛获胜的消息很快传遍了整个学校。
析：they had won the game说明The news的全部内容，因此该句为同位语从句。
（二）、运用1.如同位语从句意义完整，用that引导同位语从句。
（that 不充当任何成分，只起连接作用）例：The general gave the order that the soldiers should cross the river at once.将军下达了战士们立即过河的命令。
析：the soldiers should cross the river at once是the order的全部内容，且意义完整，因此应用that引导同位语从句。
2.如一般疑问句whether做同位语从句的连词，译成"是否"（if不能引导同位语从句）。
例：We'll discuss the problem whether the sports meeting will be held on time.我们将讨论运动会是否会如期举行的问题。
析：the sports meeting will be held on time一般疑问句做同位语从句，原句为：Will the sports meeting be held on time?3.如同位语是特殊疑问句做同位语从句，连词就应用when, where, how等疑问词引导例1：I have no idea when he will be back.例2：I have no impression how he went home, perhaps by bike.4.当主句的谓语较短，而同位语从句较长时，同位语从句常后置。
如：The thought came to him that maybe the enemy had fled the city. 他突然想到敌人可能已经逃出城了。
（三）、同位语从句和定语从句的区别:同位语从句和定语从句相似，都放在某一名词或代词后面，但同位语从句不同于定语从句。
同位语从句对名词加以补充说明，是名词全部内容的体现，且名词和同位语从句的引导词均不在从句中作成分；定语从句说明先行词的性质与特征，与先行词是修饰与被修饰的关系，且名词和定语从句的引导词均在从句中作成分。
区分时可以在先行词与与从句之间加一个系动词be,使之构成一个新句子，如果句子通顺且符合逻辑，则为同位语从句，反之，则为定语从句。
如：The report that he was going to resign was false. 他将辞职的传闻是假的。
因为the report was that he was going to resign 句意通顺，所以，that he was going to resign 是同位语从句。
例1. Information has been put forward ____ more middle school graduates will be admitted into universities.(NMET2001上海)A.whileB.thatC.whenD.as2. It is said that more middle school graduates will be admitted into universities，this is the information ____ has been put forward.A.whatB.thatC.whenD.as3. She heard a terrible noise,____ brought her heart into her mouth.(MET91)A.itB.whichC.thisD.that练习：1.The fact ____ she works hard is well known to us all.A.thatB.whatC.whyD.which2.The news ____ he was kidnapped surprised us greatly.A.whatB.thatC.whyD.when3.I have no idea ____ he will start.A.whenB.thatC.whatD./4.The thought ____ he might fail in the exam worried him.A.whenB.whichC.whatD.that5.The order ____ the prisoner be set free arrived too late.A.whichB.whetherC.thatD.what6.He often asked me the question ____ the work was worth doing.A.whetherB.whereC.thatD.when三、表语从句1、定义用作表语的从句叫作表语从句，它位于主句中的连系动词之后。