在句子中起名词性作用的各种从句统称为名词性从句。根据它们在句中所起的语法作用，这类从句分别称为主语从句、宾语从句、表语从句和同位语从句。本单元，就让我们先来学习一下在句子里面充当最重要的角色--主语成分的主语从句。
25.1主语从句概述
用作主语的从句叫做主语从句，它的位置和作用与陈述句基本结构中的主语相同。例如，在句子“Who will be our monitor hasn"t been decided yet.”中，主语从句是“Who will be our monitor”，充当主语的成分；而在句子“The monitor"s position hasn"t been decided yet.”
中，主语是“The monitor"s position”，这和“Who will be our monitor”的作用是一样的。
25.2主语从句的引导词
引导主语从句的关联词。
1.从属连词that或whether引导的主语从句
例如：
That the moon has no light of its own is a well-known fact.
月球自己不发光是一个众所周知的事实。
2.关系代词who (whoever)， what (whatever)， which (whichever)， whom， whose引导的主语从句
例如：
Which side will win is not clear.
哪一方获胜还不清楚。
Who will write the article has not been decided yet.
谁来写这篇文章还没有决定。
Who killed the scientist remains a question.
谁谋杀了那位科学家还是个谜。
Whom she borrowed money from still puzzled him.
她从谁那里借的钱依然困扰着他。
3.关系副词when，where，how，why 等引导的主语从句
How this happened is not clear to anyone.
这件事是如何发生的对每一个人来说还不清楚。
When he will be back depends on the weather.
他何时返回取决于天气情况。
Where the test will be given is not yet decided.
在何处举行考试还未确定。
25.3主语从句后置
（1）为了避免主语冗长，句子头重脚轻，经常用it 作形式主语，将主语从句放在后面作真正的主语。例如：
That we shall be late for the conference is certain.
→It is certain that we shall be late for the conference.
我们开会肯定要迟到了。
That the earth is round is known to all.
→It is known to all that the earth is round.
众所周知，地球是圆的。
When we"ll go to the restaurant has been discussed.
→It has been discussed when we"ll go to the restaurant.
我们何时去饭店已经讨论过了。
Which plan is better is clear now.
→It is clear now which plan is better.
哪一个计划更好现在已经很清楚了。
注意：what 引导的主语从句一般不用it做形式主语。
正：What he want is a book.
误：It is a book what he want.
他所需要的是一本书。
（2）用it 作形式主语的结构。
① It+be+形容词+that 从句。例如：
It is certain that she will come here tomorrow.
她明天将要来这里是确定无疑的。
② It+be+名词+ that 从句。例如：
It is a pity that we can"t go to Beijing with you.
很遗憾我们不能和你一起去北京。
③ It+be+过去分词+that 从句。例如：
It is said that Mr.Bill has arrived in Shanghai.
据说比尔先生已抵达上海。
④ It+不及物动词或短语+that 从句。例如：
It happened that I was out that day.
碰巧那天我出去了。
（3）主语从句中用陈述语序，后面的谓语动词用第三人称单数；“what”引导主语从句，表示复数概念（常从表语可以看出）时，谓语动词常用复数形式。例如：
What we need is only water.
我们需要的仅仅是水。
What we need are useful books
我们需要的是有用的书籍。
（4）it 引导的强调句与it 做形式主语的复合句不可以混淆。it 引导的强调句是用来对句子中某一部分加以强调的，其结构为：“It is (was)+强调部分+that (或who) ...”。如果被强调的主语是人，则可用who 或者that；如果被强调的部分是其他成分时，必须用that。例如：
I met John in the street yesterday.
昨天我在街上碰到了约翰。
It was I that (或who) met John in the street yesterday.（强调主语）
昨天是我在街上碰到了约翰。
It was John that I met in the street yesterday.（强调宾语）
我昨天在街上碰到的是约翰。
It was in the street that I met John yesterday.（强调地点状语，不可用where）
我昨天是在街上碰到的约翰。
It was yesterday that I met John in the street.（强调时间状语，不可用when）
我是昨天在街上碰到的约翰。
注意：强调主语时，从句谓语动词的人称和数要与被强调的部分一致（即与原主语一致）。
例如：
It is I that (或who) am your true friend.
只有我是你真正的朋友。
Dialogue & Analysis（此处有视频学习资料）
M1： Look， I， un， brought these for you to give to the kids when you have a chance.
M2： It"ll (it 作形式主语) take more than the occasional stuffed animal to convince those children they still have a father.
M1： I"m just doing what I know.I"m doing what you taught me.
M2： I never taught you to be a thief.
M1： No， you taught me to navigate people"s minds.But after what happened there weren"t awhole lot of legitimate ways for me to use that skill.
M2： What are you doing here， Dom？
M2： I think I found a way home.It"s a job for some very， very powerful people.People who I believe can fix my charges permanently.
R
ole Play
Watch the video one more time， try to imitate the dialogue with your partners， and then， show your performance to the other classmates.
I.选择题。
1.the house will be built will be discussed at tomorrow"s meeting.
A.If
B.Where
C.That
D.What
2.you come or not is up to you.
A.What
B.If
C.Why
D.Whether
3.makes mistakes must correct them.
A.Who
B.Anyone
C.Whoever
D.Anybody
4.team will win the match is a matter of public concern.
A.Which
B.That
C.If
D.How
5.leaves the room last ought to turn off the lights.
A.Anyone
B.The person
C.Whoever
D.Who
II.改错题。
1.Where shall we spend our holiday is not decided.
______________________________________________________________________________
2.You have made a mistake is a fact.
______________________________________________________________________________
3.That is a certain that we can win.
______________________________________________________________________________
4.If the policeman will come is not certain.
______________________________________________________________________________
5.Who leaves the room last should turn off the lights.
______________________________________________________________________________
Fun Time
Mother ： Why are you jumping up and down？
妈妈 ： 你为什么不停地跳上跳下的？
Tom ： It was a pity that I"ve just taken some medicine and I forgot to shake the bottle.
汤姆 ： 很遗憾，我刚吃完药，可我忘了先摇动瓶子了。