高中语法详解：名词性从句中的主语从句

名词性从句包括：主语从句、表语从句、宾语从句和同位语从句。引导名词性从句的连词有：

a.连词：that, whether, if(这三个词都不作从句的成分，同时，that无含义，而whether和if都表“是否”)

b.疑问代词：who, whom, whose, what, which

c.疑问副词：when, where, why, how

(一)主语从句

在复合句中充当主语的从句。

1.that引导主语从句

that引导主语从句时不同于其他的连词，既无词义也不作成分，只起单纯的连接作用，且通常不可以省。为了强调that引导的主语从句而置之于句首时，that不可省。

①That the driver could not control his car was obvious.

②That she was chosen made us very happy.

③That he will come is certain.

④That he would take the risk is true.

⑤That he should have married her isn't surprising.

★通常，that引导主语从句时用it做形式主语，把从句置于句尾，此时that有时可省。例如：

①It was obvious that the driver could not control his car.

②It made us very happy that she was chosen.

③It is certain that he will come.

④It is true that he would take the risk.

⑤It isn't surprising (that) he should have married her.

★如果以that从句为主语的句子是疑问句，就只能用先行词it结构。例如：

①Is it certain that he will come?

②Is it true that he would take the risk?

▲it做形式主语有以下几种不同的结构：

(1)it + be + 形容词+ that-从句

①It is likely that he will come.

②It is strange that she has ever trusted him.

③It is important that he(should) attend the meeting.

④It is best that he (should) go.

此句型在表示惊讶、喜悦、遗憾等感情色彩时，that从句中的谓语动词也常用(should)+ v原形。

It is strange that no one should have objected to the plan.

(2)it + be +名词+ that-从句

It is common knowledge that the whale is not a fish...

It's a pity that he should have massed the train.

(3) it + be + -ed分词 + that从句

It is said that he is a famous writer.

It is reported that there will be a storm this afternoon.

▲在表示建议、要求、命令等意义的被动结构中，that从句中的谓语动词常用(should)do。(参见虚拟语气部分)

It is suggested that the meeting(should)be held this afternoon.

It is demanded that he (should) leave at once.

(4) it + seem/happen/appear等不及物动词+that从句。(look不接that从句，它接to be结构或形容词)

It seems that he is wrong.

It appears that they are in urgent need of help.

He looks (to be) surprised/happy/sad.

2. whether引导主语从句，意为“是否”，只起连接作用，不充当任何成分。(置于句首时必须用whether引导，置于句尾时，间或可用if。)

Whether he will win the game is not clear.

Whether he will come is uncertain.

Whether she comes or not doesn't concern me.

It is not clear whether/if he will come.

It was uncertain whether he would come.