什么是主语从句？

主语从句，即在复合句中充当主语成分的句子，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。
引导主语从句的连词主要有：
从属连词：that whether
连接代词：who whoever whom whose what whatever which whichever
连接副词：when where how why
主语从句

定义
在复合句中充当主语成分的句子叫做主语从句。

第一部分
常规主语从句，即句子在复合句中充当一个主语
主语从句是名词性从句，所以它的时态受主句的影响。
（1）That he finished writing the composition in such a short time surprised us all.
（2）Whether we will go for an outing tomorrow remains unknown.
（3）Who will be our monitor hasn't been decided yet.
（4）Whom we must study for is a question of great importance.
（5）What caused the accident remains unknown.
（6）Whatever you did is right.
（7）Who the watch belongs to is unknown.
（8）What we need is time.
（9）What we need are good doctors.
名词性从句的时态规则：
1）、主句谓语动词是现在或将来时，从句谓语动词可以用任何需要的时态。
　　I wonder what he is doing now.
　　Do you know when and where he was born?
　　I'll write to tell him where I 'll meet him.
2）、主句谓语动词是过去时，从句谓语动词一般需用过去时。
（1）从句与主句的谓语动作同时发生，从句用一般过去时或过去进行时。
　　I thought that he studied hard.
　　He told me his son was watching TV.
（2）从句谓语动作在主句谓语动作之后，从句用过去将来时。
　　He said he would spend his holidays in Qingdao.
（3）从句谓语动作在主句谓语动作之前，从句用过去完成时。
　　They told me they had waited for me half an hour.
（4）从句如果有表示过去某一具体时间的状语，仍用过去时。
　　She told me her brother died in 1945.

　　3)客观的条件，从句时态不受丝毫影响。
（1）从句说明的是不受时间限制的事实或真理（客观真理）。
　　The teacher told us that the earth turns from west to east.
　　He said Asia is the largest continent.
It was true that water freezes at thirty-two degrees Fahrenheit.
（2）当从句表示的是反复出现的时态不变。
　　She told me that she gets up at six o'clock every morning.
　　He asked me when the train usually starts.
（3）当从句是用来表达解释科技内容时时态不变。
　　.....
　　（4）当从句的谓语动作仍在继续时，或者表示现在仍在的状态时，时态不变。
　　He said that the workers in that factory are trying to fulfill the task in time.
　　She told me the other day that she is only 10.
（5）当从句的动作状态还未发生，要用一般式表示。强调与现在的联系。
　　He said he will wait for me this evening.
小结:
（1）引导主语从句连词有that,whether,who,what,whatever等
（2）连词位于句首不能省略
（3）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）
注意：当what引导的名词性分句作主语时，主谓一致问题极为复杂。著名学者周海中教授在论文《关于what-分句作主语的主谓一致问题》中就这一问题做了深入研究，并给出了10种的主谓一致关系。这些一致关系值得英语学习者和使用者特别注意。[1]

第二部分
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
（1）It is certain that he will win the match.
（2）It is true that he has made a very important discovery in chemistry.
（3）It is very likely that they will hold a meeting.
（4）It is strange that he should do that.
（5）It is important that we all should attend the meeting.
（6）It is strange that the man should have stuck to his silly ideas.
（7）It is a pity that we won't be able to go to the south to spend our summer vacation.
（8）It is still a mystery what caused the accident.
（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)
（10）It is known to all that the gun powder was first invented by the Chinese.
（11）It is suggested that the work should be done with great care.
（12）It happened that the two cheats were there. (=The two cheats happened to be there)
小结:
（1）为避免头重脚轻，常以形式主语it引导从句。 It+形式主语谓语+从句。
（2）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.

用法
主语从句与宾语从句两者的用法和在句中的作用是不同的，下面从它们的各自用法来介绍。
一．主语从句
主语从句 定义：在句子中担当主语的是一个从句，这个从句就叫做主语从句。
主语从句是在复合句中充当主语的从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。
1. 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 Prof.Lin 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，再把谓语动词后面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 则不然。例如：
What you said yesterday is right.
二：宾语从句用以区分主语从句的几个特征：
1、引导词：what\which\whose\when\whether\if\where
2语序：宾语从句必须是用陈述语句。（名词性从句都是陈述语序）
如：I think that you must work harder.
宾语从句的引导词、连接词的区别、否定转移等现象。
补充：从句的语序永远是陈述句。

在复合句中充当主语成分的句子叫做主语从句.
常规主语从句，即句子在复合句中充当一个主语
主语从句的时态：不受主句的时态影响和限制。
（1）That
he
finished
writing
the
composition
in
such
a
short
time
surprised
us
all.
（2）Whether
we
will
go
for
an
outing
tomorrow
remains
unknown.
小结:
（1)引导主语从句连词有that,whether,who,what,whatever等
（2)连词位于句首不能省略
（3)主语从句大多数情况下视为第三人称单数
第二部分
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
（1）It
is
certain
that
he
will
win
the
match.
（2）It
is
true
that
he
has
made
a
very
important
discovery
in
chemistry.
（3）It
is
very
likely
that
they
will
hold
a
meeting.
小结:
（1）为避免头重脚轻,常以形式主语it引导从句.
It+形式主语谓语+从句.
（2）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.

主语从句(Subject Clause)

定义:如果一个句子在复合句中充当一个主语,那么这个句子就是主语从句.

第一部分：常规主语从句，即句子在复合句中充当一个主语

（1）That he finished writing the composition in such a short time surprised us all.

（2）Whether we will go for an outing tomorrow remains unknown.

（3）Who will be our monitor has't been decided yet.

（4） Whom we must study for is a question of great importance.

（5）What caused the accident remains unknown.

（6）Whatever you did is right.

（7）Whose watch was lost is unknown.

（8）What we need is time.

（9）What we need are good doctors.

小结:

（1)引导主语从句连词有that,whether,who,what,whatever等

（2)连词位于句首不能省略

（3)主语从句大多数情况下视为单三,但也有例外,如例9

第二部分：为了防止句子头重脚轻，通常把形式主语it放在主语位置，真正主语搁置于句末

（1）It is certain that he will win the match.

（2）It is true that he has made a very important discovery in chemistry.

（3）It is very likely that they will hold a meeting.

（4）It is strange that he should do that.

（5）It is important that we all should attend the meeting.

（6）It is strange that the man should have stuck to his silly ideas.

（7）It is a pity that we won't be able to go to the south to spend our summer vacation.

（8）It is still a mystery what caused the accident.

（9）It is said that he has gone to shanghai.(=He is said to have gone to shanghai)

（10）It is known to all that the gun powder was first invented by the Chinese.

（11）It is suggested that the work should be done with great care.

（12）It seems that he has seen the film.(=He seems to have seen the film)

（13）It happened that the two cheats were there. (=The two cheats happened to be there)

小结:

（1)以that 引出的主语从句,常以形式主语it引导. It is +形容词/名词/某些动词ed + that 从句.

（2) 在有些that从句中要用虚拟语气 (should+do/should+have done)例句4,5,6,11.

定义:如果一个句子在复合句中充当一个主语,那么这个句子就是主语从句.
例如：（Whatever you did ）is right.括号部分做主语，is系动词，right则做表语，这就是一个主语从句。前提是在复合句中，而且是一个句子充当这个复合句的主语。
（Whether we will go for an outing tomorrow） remains unknown.
这就是一个主谓宾结构的复合句，括号部分充当整个句子的主语。
还有一类，如：It is certain that he will win the match.
It is true that he has made a very important discovery in chemistry 这类句子中，为了避免句子头重脚轻，it充当形式主语，真正的主语是that后面的整个句子 。这是一类特殊的主语从句，我觉得也是最好掌握的。它的大体形式是it is +adj./n./v-ed +that从句

比如句子：What
I
want
to
say
is
"I
love
you"其中What
I
want
to
say整体是句子的主语，is是系动词，当然，“
I
love
you”整体自然而然就是表语了！！由于语句的主语是一个what引导的从句，所以这个句子就是一个典型的主语从句！！！！