主语从句、表语从句、宾语从句、同位语从句在复合句中起名词的作用,这四种从句合称名词性从句.它们在主句中充当主语、表语、宾语或同位语,都不能用逗号与主句分开.(注：句中划线部分为从句)
■主语从句
1、定义：先找到句子的谓语,主句的谓语前面的从句,叫做主语从句.
Why he left wasn't important.(wasn't是主句的谓语动词,why he left做主语从句)
That she became an artist may have been due to her father's influence.(may have been due是主句的谓语动词,that she became an artist做主语从句)
2、借助形式主语it,主语从句放在后面
①由that引导的主语从句可以放在主语的位置,这时that不可以省略.例如：That prices will go up is certain.
that主语从句放在句首的情况不是很常见,绝大部分主语从句都借助形式主语it,主语从句放在后面.
It is strange that he knows nothing about it.
It is a pity that he can't swim.
It happened that he wasn't in that day.
It is said that there has been an earthquake in India.
在口语中,用it做形式主语时,主语从句的that可以省略.
②wh-疑问词引导的主语从句,可以放在句首(第一组),也可以借助形式主语it(第二组).
第一组：When he'll be back depends much on the weather.
How it was done was a mystery.
Whether we'll succeed remains to be seen.
第二组：It is uncertain whether the game will be held.
It's a puzzle how life began.
It doesn't matter much where we live.
Is it known where he went?
■宾语从句
1、定义：及物动词(第一组)和介词(第二组)的后面可以接从句做宾语,叫做宾语从句.
第一组I guess(that) we'll leave soon.
He asked when we would be in London.
She informed me (that) she was to send for it the next day.(that可以省略)
I'll tell you what I read in today's paper.
第二组
I'll find out whether she's interested in going.
Can you give us a description of what has happened.
She was shocked by what she had seen.
2、借助形式宾语it,that引导的宾语从句放在后面.I've heard it said that you have won a scholarship.
分析：it在句中做形式宾语,said做宾语补足语,that you have won a scholarship是真正的宾语.She has seen to it that all the children are well taken care of.
分析：it在句中做形式宾语,that all the children are well taken care of是真正的宾语.
■表语从句
1、定义：系动词的后面可以接句子做表语,叫做表语从句.
The fact is that she never liked him.
That is not what I meant.
The question is who is responsible for what has happened.
She is no longer what she used to be.
2、注意：
①The reason Hollywood was a good place for making movies was that the sun shines there every day.
分析：the reason做主语时,表语从句用that引导.
②引导表语从句的that一般不省略.
■同位语从句
1、定义：有些名词的后面可以跟that引导的从句,说明这个名词的内容,这样的从句叫做同位语从句.所以,只有可以有内容的名词才可以接同位语从句,例如：fact,message,idea,possibility,conclusion,thought,news,order,report,rumor等等.
that在从句中不做成分,但是不能省略.
He had the feeling that he would not see her again.I've come to the conclusion that it won't be wise to do so.
2、有时为了保持句子的平衡,同位语从句有时和前面的名词分开.
The rumor spread that a new school would be built here.
that引导的同位语从句修饰the rumor,但是主句The rumor spread太短,如果把后面的同位语从句放在the rumor后面,主语太长,给人头重脚轻的感觉.
3、在少数情况下,名词性从句的其他关联词也可以引导同位语从句.但if不可以引导同位语从句.
You have no idea how worried I was.
I have no idea why she left.
There is some doubt whether John will come on time.
在学习名词性从句时注意下面四点：
一、名词性从句要用陈述语序,也就是说：①名词性从句的关联词做主语的话,关联词后面接谓语动词；②名词性从句的关联词不做主语的话,关联词后面接从句的主语和谓语.
①I don't know who broke the window.(宾语从句中关联词who做主语,broke是谓语动词)
②The little boy told his mother where he found the coin.(宾语从句中关联词where不做主语,where后面接主谓部分he found)
二、感叹句做名词性从句时语序不做变化
Don't you know how excited I was then?
三、时态的呼应
某些从句(特别是宾语从句)中的动词时态,常受主句谓语时态的制约.
①如果主句谓语动词为现在时或将来时,从句中的谓语可以不受影响.例如：
Has he told you when he received the gift?
Will you tell me how you two are getting along?
②如果主语谓语动词为过去时,从句谓语一般要跟着改为相关的过去时.
I didn't know where they lived.He thought he was working for the people.asked him how long he had been waiting.
③但是,当宾语从句表示永恒真理时谓语可以不变：This proved that the earth is round.
四、当and连接两个that引导的宾语从句时,and后面的that不可以省略
She promised (that) she would come and see him sometimes and that she would never forget him.句中的第一个that可以省略,为了清楚地表示出第二个that引导的是宾语从句,第二个that不省略.
