主语从句
概念
主语相当于句子的话题或主题，即要说明的入或物，也可以说是动作的发起者（主动语态中）。

能用来作主语的有名词、代词、动名词、动词不定式、主语从句等。

主语从句，即在复杂句中充当主语成分的句子，主语从句分为常规主语从句和后置主语从句。

例如"That he finished writing the composition in such a short time surprised us all"，他在很短的时间内写完作文让我们都很惊讶，此复合句主要描述的是某事让我们惊讶，某事就是该复合句的主语，因此“That he finished writing the composition in such a short time”便是该复合句的主语从句。

连词：连词的选择同总论中所述。

分类：
一、常规主语从句
句子在复合句中充当一个主语
时态：主语从句的时态不受主句时态的影响和限制
人称：主语从句大多数情况下视为第三人称单数
（1）That he finished writing the composition in such a short time surprised us all.
他在如此短的时间内写完作文使我们十分惊讶。

（2）Whether we will go for an outing tomorrow remains unknown.
我们明天是否会去郊游仍旧未知。

（3）Who will be our monitor hasn't been decided yet.
现在还没有决定谁会成为我们的班长。

（4）Whom we must study for is a question of great importance.
“我们为谁而学习”是个很重要的问题。

（5）What caused the accident remains unknown.
事故的起因是未知的。

（6）Whatever you did is right.
你做的任何事都是正确的。

（7）Whom the watch belongs to is unknown.
手表的归属是未知的。

（8）What we need is time.
我们需要的是时间。

（9）What we need are good doctors
注意：主语从句大多数情况下视为第三人称单数，但也有例外，如例（9）
二、后置主语从句
为了防止句子头重脚轻，通常把形式主语it放在句首位置，真正主语搁置于句末，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句。

例句：
（1）It is certain that he will win the match.
他一定会赢得这场比赛的。

（2）It is true that he has made a very important discovery in chemistry.
他的确在化学领域有了一个伟大的发现。

（3）It is very likely that they will hold a meeting.
他们很有可能会举行一场会议。

（4）It is strange that he should do that.
他竟然那样做是很奇怪的。

（5）It is important that we all should attend the meeting.
重要的是我们都应该参加会议。

（6）It is strange that the man should have stuck to his silly ideas.
真奇怪，这个人竟然坚持自己愚蠢的想法。

（7）It is a pity that we won't be able to go to the south to spend our summer vacation.
真遗憾我们不能去南方度过我们的暑假。

（8）It is still a mystery what caused the accident.
造成事故的原因仍旧是个秘密。

（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
据说他已经去上海了。

（10）It is known to all that the gun powder was first invented by the Chinese.
所有人都知道火药最初是由中国人发明的。

（11）It is suggested that the work should be done with great care.
建议工作应该被小心地被完成。

（12）It happened that the two cheats were there. (=The two cheats happened to be there)
两个骗子碰巧在那里。

注意：由于主语从句一般被认为是单数形式，形式主语的谓语动词、be动词+表语均
为单数第三人称形式或过去式。

It 作形式主语和it引导强调句的比较
①主语从句常用it作形式主语，一般常用句型为：It is+{名词/形容词/过去分词}+主语从句。

例如：
It is still a question whether she will come or not.
It is strange that you should like him.
It is still unknown which team will win the match.
另外，还有一些比较多见的结构：
It turned out that……;
It has been proved that……;
It happened/occurred that……;
It is well-known that……等等
②而强调句则不同，它的结构是：It+be+被强调部分+that+从句
强调主语：It is the times that produce their heroes.时势造英雄
强调宾语：It is English that teaches us.
强调状语：It was in shanghai that I saw the film.
判断是否是强调句有一个方法，就是将that以后的“句子的其余部分”拿出来单独看，看有没有缺成分，缺的是什么成分，再把be动词后面that前面的那部分，带到你认为缺成分的地方，如果放进去是一句完整的句子了，那就说明是强调句。

2. 用it 作形式主语的结构
(1) It is +名词+that从句
It is a fact that …事实是……
It is an honor that …非常荣幸
It is common knowledge that …是常识
(2) it is +形容词+that从句
It is natural that…很自然……
It is strange that…奇怪的是……
(3) it +不及物动词+that从句
It seems that…似乎……
It happened that…碰巧……
(4) it is+过去分词+that从句
It is reported that…据报道……
It has been proved that…已证实……
3.主语从句不可位于句首的五种情况
(1) if引导的主语从句不可居于复合句句首。

(2) It is said , (reported) …结构中的主语从句不可提前。

例如：
It is said that President Jingo will visit our school next week.
(3) It happens…, It occurs…结构中的主语从句不可提前。

例如：
It occurred to him that he failed in the examination.
(4) It doesn’t matter how/whether …结构中的主语从句不可提前。

例如：
It doesn’t matter whether he is wrong or not.
(5) 含主语从句的复合句是疑问句时，主语从句不可提前。

例如：
Is it likely that it will rain in the evening
4. What 与that 在引导主语从句时的区别
What 引导主语从句时在从句中充当句子成分，如主语．宾语．表语，而that 则不做任何成分。

例如：
What you said yesterday is right.
That he has gone made me sad. （that位于句首，不做任何成分，但不可省略）。