主语从句与先行it
主语从句可以按其引导词的不同分为三类：
第一类，用从属连词that引导的主语从句，例如：
That we shall be late is certain.
That the driver could not control his car was obvious.
这种结构主要是对that从句的内容进行强调，属正式文体，连词that不可以省略；但是在一般情况下，往往使用先行it结构，即用it作形式主语，而把that从句放到后面，这时，在口语中，连词that有时则可以省略。所以上述两句可以改为：
It is certain that we shall be late.
It was obvious that the driver could not control his car.
如果整个句子是疑问形式，就只能用先行it结构，例如：
Is it true that he would take the risk？
Is it possible that they will come tomorrow？
常使用先行it结构，用that从句作主语的句子有下列几个句型：
1、It + be + 形容词 + that从句：
It is clear that he was telling the truth.
It’s probable that we’ll be a little late.
2、It + be + 名词词组 + that从句：
It’s a pity that you can’t go with us.
3、It + 及物动词 + 宾语 + that从句：
It worried her a bit that her hair was turning grey.
It shocked me that Peter didn’t tell anybody where he was.
4、It + be + 过去分词 + that从句：
It is said that he has been there many times.
5、It + seem/happen/appear等不及物动词 + that从句：
It seems that he has lost something.
注意：
在上述第1和第2两种句型中，that从句前置与使用先行it，that从句后置在意义上没有什么差异；但使用先行it结构较为常见。
在第3种句型中，that从句前置在语法上是可能的，但实际上并不常见，通常总是使用先行it结构 .
第4种句型实质上是被动结构，由于that从句不可以位于句首作被动句的主语，所以只能使用先行it结构。
第5种句型已经形成了固定的搭配关系，that从句不能前置，只能使用先行it结构，不过，这种结构可以转换为带有不定式的简单句，例如：
It happened that I had no money with me that day.
→ I happened to have no money with me that day.
第二类，用连接代词who、whose、which、what，连接副词when、where、how、why，以及连词whether（或if）引导的主语从句，例如：
Who was responsible for the accident is not yet clear.
→ It is not yet clear who was responsible for the accident.
What he did is not yet known.
→ It is not yet known what he did.
Whether it is true remains a problem.
→ It remains a problem whether / if it is true.
从以上例句可以看出，这类主语从句可以直接放在句首作主语，也可以使用先行it结构，把主语从句放在后面，两种结构可以互换，意义上无差异，但用if 引导主语从句时，只能采取先行it结构，也就是说if不可以引导置于句首的主语从句。
第三类，用what、whatever、where、wherever、whoever、whichever等代词引导的主语从句，例如：
What he said at the meeting encouraged everyone.
Whoever fails to see this will make a big blunder.
这类从句一般相当于带有定语从句所修饰的名词词组，即在结构上相当于一个名词加上一个定语从句，例如：
What he wants to buy is a ten-speed bicycle.
→ The thing that he wants to buy is a ten-speed bicycle.
Whoever breaks the law is sure to be punished.
→ Anyone who breaks the law is sure to be punished.
切记，这类主语从句不可以使用先行it结构。
上述第二类主语从句与第三类主语从句尽管在形式上十分相似，但实际上是不同的。第二类结构中用连接代/副词引导的从句系由特殊疑问句转化而来，以whether/if引导的从句系由一般疑问句转化而来，自然含有疑问的意味，例如：
When they will have the sports meet is still a question.
→ When will they have the sports meet？
Who he is doesn’t concern me.
→ Who is he？
Whether he will join us won’t make too much difference.
→ Will he join us？
而第三类结构中的主语从句则没有疑问的意味。
试比较下列各句：
① What caused the accident is a complete mystery.
② What caused the accident was a broken bottle.
③ What she looks like doesn’t matter.
④ What she’d like is a digital watch.
上述各句中尽管都有一个以what引导的主语从句，但其意义不尽相同；第①和③句中的主语从句系由“What caused the accident？”和“What does she look like？”转化而来，所以可以改成：
It is a complete mystery what caused the accident.
It doesn’t matter what she looks like.
第②④句中的主语从句含义分别为“The thing that caused the accident”和“The thing which she’d like”，所以不含疑问意味，因此不可以说：
It was a broken bottle what caused the accident.
It is a digital watch what she’d like.
但是可以说：
It was a broken bottle that caused the accident.
It is a digital watch that she’d like.
不过，这已不是主语从句，而是强调结构了。
《高中英语语法-主语从句与先行it》由留学英语组编辑整理(m.liuxue86.com)