主语从句的引导词用法

在句子中担当主语的从句，就叫主语从句。根据主语从句所缺句子成分的情况，主语从句会由不同的连接词连接，常见的连接词有：that、if/whether、连接代词、连接副词和what等。

一、that引导的主语从句

(一)当主语从句本身不缺成分时，就会使用that来连接。此时，that只起连接作用，但不能省略。如：

That he will come to our village tomorrow surprises us all. (他要来我们村，让我们都感到很惊讶。)

That she used to be a spy is known to all. (大家都知道，她过去是个间谍。)

这种情况下，因为主语从句往往比谓语部分更长，会导致整个句子头重脚轻，所以经常会用it作为形式主语来替换主语从句，而把主语从句放到句末。此时，that也只起连接作用，但可以省略。比如，以上两个句子就存在这个问题，所以一般都会改为：

It surprises us all (that) he will come to our village tomorrow.

It is known to all (that) she used to be a spy.

(二)如果句子是疑问句，则必须用带形式主语it的结构。如：

That she is our English teacher is true. (她是我们的英语老师，是真的。)

That he has been there twice is a fact. (他去过那儿两次了，是真的。)

这两句话如果改为一般疑问句时，不能这么改：

错误：Is that she is your English teacher true?

错误：Is that he has been there twice a fact?

而是要改为含有形式主语it的句子：

Is it true that she is your English teacher?

Is it a fact that he has been there twice?

(三)在形式主语it引导的主语从句结构

“It is+形容词/名词/名词短语等+that主语从句”中，如果is后面的表语是表示重要或必要等(如important、necessary、a necessity等)含义时，that从句中的谓语动词一般要采用“should+do”的虚拟语气形式，should经常省略。如：

It is imperative that we (should) make a quick decision. (我们要尽快做出决定。)

It is necessary that we (should) learn English well. (我们必须学好英语。)

二、if/whether引导的主语从句

当主语从句本身含有是否的意思时，连接词必须用if或whether。但是，如果主语从句位于句首时，只能用whether，不能用if。如：

Whether it will do good to us remains to be seen. (是否对我们有害还要看一看。)

这种情况下，因为主语从句也比较长，会导致头重脚轻的情况，所以也经常用形式主语it来代替，而把主语从句放到句末去。此时，连接词除了用whether外，也可以用if。如上面的句子就可以改为：

It remains to be seen if/whether it will do good to us.

再比如：

Whether the band would visit our city was still a mystery. (这个乐队是否会来我们城市仍然是个谜。)

上面的句子可以改为：It was still a mystery if/whether the band would visity our city.

三、连接代词引导的主语从句

常用来连接主语从句的连接代词有：who、whom、whose、which、whoever、whomever、whichever等。这些连接代词在从句中有的作主语，有的作宾语，有的作定语。如：

作主语：Who won the game hasn't been announced yet. (谁赢得了比赛，还没宣布。)

作宾语：Whom you talked to just now will be confirmed later. (你刚才跟谁说话了，之后会证实的。)

作定语：Whichever nation explodes an atomic bomb will engage the whole world in war. (无论哪个国家使用原子弹都会使全世界卷入战争。)

为了避免头重脚轻，这种情况下的主语从句也经常用形式主语it来代替，而把主语从句放到句末。但是，连接代词始终都不能省略。如上面的两句话可以改为：

It hasn't been announced yet who won the game.

It will be confirmed later whom you talked to just now.

It will engage the whole world in war whichever nation explodes an atomic bomb.

四、连接副词引导的主语从句

能够用来连接主语从句的连接副词主要有：when、where、why、how等。这些连接副词在主语从句中作状语。如：

When they will arrive at the station doesn't matter. (他们何时到站没有什么关系。)

How the accident happended is still unknown. (这场事故是怎么发生的，还不清楚。)

Why he was absent from the meeting remains a mystery. (他为什么没有参加会议仍然是个谜。)

Where I will spend my summer is none of your business. (我在哪里过暑假不关你的事。)

为了避免头重脚轻，上面的句子可以改为：

It doesn't matter when they will arrive at the station.

It is still unknown how the accident happended.

It remains a mystery why he was absent from the meeting.

It is none of your business where I will spend my summer.

五、what引导的主语从句

与that不同的是，what除了可以引导主语从句起连接作用外，还同时在从句中充当主语或宾语。如：

作主语：What really matters is that she needs a shower at once. (真正重要的是，她得马上冲个澡。)

作宾语：What I am worried about is their taking my son to France. (我担心的是他们要把我儿子带到法国去。)

有的时候，whatever也可以用来连接主语从句。如：

Whatever you do makes no difference to me. (你做什么对我来说都没区别。)

需要注意的是，当what或whatever引导主语从句时，一般不能改为由形式主语it来引导的句子。