在主从复合句中，从句可以充当主句的主语、表语、宾语或同位语。由于在多数情况下，主语、表语、宾语或同位语这四种句子成分由名词性词类充当，所以，我们把这些作用相当于名词的从句统称为名词性从句，把充当主语、表语、宾语或同位语的从句分别称为主语从句、表语从句、宾语从句或同位语从句。也就是说充当什么成分就叫什么从句。名词性从句由连接词(或关联词)引导。

常用的连接词有：

疑问词(who, whom, whose, what, which, where, why, when, how)可以引导主语、宾语和表语从句。

它们的特点是：1、疑问词有本身的词义;2、疑问词在从句中担当句子成分，如主语、宾语或状语;3、这种疑问词引导的从句一律用陈述语序，不能用疑问语序。

For example:

Do you know whom they are looking for? 你知道他们在找谁吗?(宾语从句，陈述语序。不能是whom are they looking for?)

I don’t know who did it. 我不知道这是谁干的。(宾语从句。在从句中如果疑问词作主语，其陈述语序和疑问语序一致。)

She asked me where I had been. 她问我到哪儿去了。(宾语从句。陈述语序，不能是where had I been. 直接引语，间接引语。注意这句话的时态。)

Can you tell me when the train will arrive? 你能告诉我火车什么时候到吗?(宾语从句。陈述语序，不能是when will the train arrive. 直接引语，间接引语。)

I don’t know why he hasn’t come yet. 我不知道他为什么还没来。(宾语从句，陈述语序。不能是why hasn’t he come yet.)

He didn’t tell me what you were doing. 他没和我说你在干什么。(宾语从句，陈述语序。不能是what were you doing.)

What you have done might do harm t other people. 你所做过的事情有可能伤害别人。(主语从句。what 作从句的宾语。陈述语序，不能是what have you done. 如果用疑问语序，意思发生了变化，成了“你做了什么?”。)

I don’t know where he is now. 我不知道他现在在哪里。(宾语从句，陈述语序。不能是where is he now.)

Where he went for his weekend is not known. 不知道他去哪里过的周末。(主语从句，where 作从句的地点状语。陈述语序，不能是where did he go for his weekend.)

Whose fault this is is not important. 这是谁的过错并不重要。(主语从句，whose 作从句的定语。陈述语序，不能是whose fault is this.)

What I want to know is where he has gone for his weekend. 我想知道的是他到哪里度周末去了。(这句话包含两个名词性从句：what I want to know是主语从句，what 在从句中作宾语，从句用陈述语序，不能是What do I want to know. Where he has gone for his weekend 是表语从句，where 在从句中作状语，从句用陈述语序，不能是where has he gone for his weekend.)

下面分述主语从句、宾语从句、同位语从句和表语从句。

A主语从句

1.that 引导主语从句时，that 没有意义，但不能省略。(that 引导宾语从句时可以省略。)

For example:

It worried her a bit that her hair was turning gray. 她的头发正在变白，这使她很不安。that引导主语从句，it 作形式主语，that 不能省略。

That she is a rich woman is known to us all. 众所周知，她是个富有的女人。这是that 引导主语从句，that 不能省略。这句话可以改为用it 作形式主语的句型。请同学们改写：It is known to us all that she is a rich woman.)

We didn’t know (that) you had sold your house. 我们不知道你已经把你的房子卖了。这是that引导宾语从句，that 可以省略。注意这句话的时态。

2.从句作主语时，多数情况下由 it 作形式主语，而把主语从句放在后面，尤其是谓语部分(包括宾语)较短的情况下。

For example:

It wasn’t very clear what she meant. 不清楚她是什么意思。

It is important that he should come on time. 他按时来是很重要的。

It is true that that man on the left is a well-known writer here. 左边那个人是本地的一位著名作家，这是真的。这里，第一个that 引导主语从句，是单纯的连词，无词义，不作句子成分。第二个that 是指示代词，作句子成分，是man 的定语。

3.whether 既可以引导主语从句也可以引导宾语从句，但if不能引导主语从句。whether 后面可以加or not, 而if 不能与or not 连用。作介词宾语时不用if.

For example:

Whether I knew John doesn’t matter. = It doesn’t matter whether I knew John. 我是否认识约翰没有关系。

Whether or not she’ll come isn’t clear. = Whether she’ll come or not isn’t clear. = It isn’t clear whether …. 她是否来还不清楚。

It all depends on whether we can get their cooperation. 这是主语从句还是宾语从句?it 是形式主语吗?it 是代词，whether 引导的是宾语从句，作介词on 的宾语，不能用if引导。请翻译这句话。这完全取决于我们是否能得到他们的合作。

I worry about whether I hurt her feelings. 宾语从句，介词宾语，不能用if 引导。请翻译这句话。

She asked me whether/if you were married. 宾语从句，既可以用whether, 又可以用if 引导。注意时态的对应。请翻译这句话。

We haven’t decided whether/if we shall give them aid.

I’m not sure whether/if the report is believable.

Now let’s do some translation:

现在还很难预测谁会赢得下一届总统选举。

It is still hard to predict who will win the next presidential election.

对于多数公民来说，谁当选总统无关紧要。

Who will become the president doesn’t matter much to most citizens.

这位前总统是否会被判处死刑还有待观察。It remains to be seen whether the ex-president will be sentenced to death.

真可惜，王教授不能出席我们的英语晚会。

It is a pity that Prof. Wang can’t attend our English Evening.

我们需要的是更多的时间。

What we need is more time.