六、名词性从句
　　名词性从句的功能相当于名词词组。按其句法功能，名词性从句可分为主语从句、表语从句、宾语从句和同位语从句。引导名词性从句的有：连接词that, whether和if，关系代词who, whom, whose, what, which, wahtever, whichever, whoever以及关系副词when, where和how。
　　主语从句(The Subject Clause)
　　1. 主语从句在句中作主语。如：
　　Which team will win the match is still not certain.
　　Why he did that wasn’t quite clear.
　　2. 主语从句位于句首时，会使句子显得笨重，因此常以it作为形式主语，而将主语从句后置，以保持句子平衡。如：
　　It is strange that she didn’t come yesterday.
　　3. whether与if引导主语从句的区别：
　　whether引导的主语从句既可放在句首也可放在句尾，if引导的主语从句只能放在句末。如：
　　Whether she comes or not makes no difference.
　　It is doubtful whether / if the President knew the details of the plan.
　　表语从句(The predicative clause)
　　1. 表语从句在句中作表语，位于主句中的系动词之后。如：
　　The question is whether she can speak English.
　　That is what we need.
　　2. 表语从句可用whether引导，也可用as if引导，但不能用if引导。如：
　　His first question was whether Tom had arrived yet.
　　It looked as if it was going to rain.
　　3. 主句的主语是reason时，表语从句要用that，不能用because。如：
　　The reason why he was dismissed was that he was careless and irresponsible.
　　同位语从句(The Appositive Clause)
　　1. 同位语从句在句中作同位语，它一般要放在主句中某些名词的后面，说明这些名词的含义。可跟同位语从句的名词主要有fact, news, promise, reason, idea等。如：
　　We heard the news that our team had won.
　　I have no idea how soon they are coming.
　　2. 同位语从句与定语从句的主要区别：
　　从意义上来说，同位语从句对一个名词加以补充说明，定语从句对一个名词进行修饰或限定。从结构上说，同位语从句由连词引导，连词在从句中不作任何成分，只起引导作用;引导定语从句的关系代词在从句中担任一定成分，含有代替所修饰或限制词语的意义。如：
　　They were all shocked at the news that Germany had declared war on Russia. 他们都为德国向俄国宣战而感到震惊。(同位语从句，that只起引导作用，在句中不作任何成分。)
　　They were all shocked at the news that was announced on the radio. 他们都为收音机里宣布的消息而震惊。(定语从句，that在句中作主语，若省略that，句子成分不全。)
　　宾语从句(The Object Clause)
　　1. 宾语从句在句中作及物动词或介词的宾语。如：
　　I wonder who he is.
　　He is sorry for what he has done.
　　注意：在口语及非正式文体中，that常省略。
　　2. 使用宾语从句时应注意时态呼应。
　　1)主句谓语动词是现在时或将来时，从句谓语动词可以用任何所需要的时态。
　　2)主句谓语动词是过去时态，从句谓语动词一般须用过去时。如：
　　I thought he studied hard.
　　3) 从句如果与某一具体的过去时间状语连用，尽管其谓语动作发生在主句谓语动作之前，仍用一般过去时。如：
　　She told me that her father died in 1991.
　　4) 从句所说明的是一般真理或客观事实，即使主句用过去时，从句仍用一般现在时。
　　The teacher told the students that the computer is a wonderful machine.
　　3. 不能省略that的宾语从句
　　1)句中的动词后接两个或两个以上的that从句作宾语时，第一个that可省，但后面的that不可省。如：
　　He said (that) he couldn’t tell you right away and that you wouldn’t understand.
　　2) 当主句的谓语动词与that宾语从句之间有插入语时，that一般不可省。如：
　　Just then I noticed, for the first time, that our master was wearing his fine green coat and his black silk cap.
　　3) 当that从句是双宾语中的直接宾语时，that不可省。如：
　　I must never tell anyone that I could not see the cloth.
　　4) 当that宾语从句前有it作形式宾语时，that不可省。如：
　　We think it important that we study hard.
　　4. whether和if引导的宾语从句可以互换使用，但介词后的宾语从句只能用whether，不能用if。如：
　　He asked me if / whether my brother was at home.
　　It is a question of whether we should go.