it作形式主语

有时为了考虑句子平衡，通常在主语从句处使用形式主语it，而将真正的主语从句移至句末。这分四种情况：
1. 对于以连词that引导的主语从句，通常用形式主语it替代主语从句；
2. 对于以连接代词（副词）引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句；
3. 对关系代词型what引导的主语从句，通常直接将主语从句放在句首；
4. 如果句子是疑问句，则必须用带形式主语it的结构。

四种基本句型

1. 连词that引导的主语从句，通常用形式主语it替代主语从句。
(1) It is +名词+从句
It is a fact that... 事实是……；
It is good news that... 是好消息；
It is a question that... 是个问题；
It is common knowledge that... 是常识
类似的名词还有：a pity, a wonder, a good thing, no wonder, surprise等。
it is no surprise 毫不惊奇地
例如：
It is a mystery to me how it all happened.
这是怎么发生的，对我来说就是一个谜。
It is common knowledge that the whale is not a fish.
鲸鱼不是鱼，这是一个常识。
It is no surprise that China is making progress on such fronts: the manufacture of large passenger aircraft and high-speed trains, research in yeast and aerospace.
中国在很多尖端领域，如大飞机制造，高铁、干细胞、航天航空研究等领域取得飞速发展一点都不惊讶。
(2) It is +形容词+从句
It is necessary that... 有必要……；
It is clear that... 很清楚……；
It is likely that... 很可能……；
It is important that... 重要的是……
类似的形容词还有：strange; natural; obvious; true; good; wonderful; possible; unlikely; quite; unusual; certain; evident; worth-while; surprising; interesting; astonishing, etc.
例如：
It is doubtful whether she will be able to come.
我们很怀疑她是否能来。
It is essential that he should be here by the weekend.
周末他务必要来这里。
It seems obvious that we can not go on like this.
很明显，我们不能再这样继续下去。
It is necessary that you (should) master the computer.
你必须要掌握电脑技术。
(3) It is +过去分词+从句
It is said that... 据说……；
It is reported that... 据报道……；
It has been proved that... 已证明……；
It must be proved that... 必须指出……
类似的过去分词还有：known; estimated; expected; believed; thought; hoped; noted; discussed; required; decided; suggested; demanded; made clear; found out等
例如：
It used to be said that women must do twice as well as men to be thought half as good.
过去人们说女性必须做两倍于男性的工作才能获得一半于男性的好评。
It has not been made clear when the new road is to be opened to traffic.
目前还不清楚，该路段何时可通行使用。
(4) 其他
It seems (happened / appears / doesn't matter / makes no difference / occurred...) that... 主语从句不可提前
例如：
It makes no difference whether he will attend the meeting or not.
他是否出席会议并没有什么不同。
For now, it appears that the dream might have to wait for his great-grandchildren.
从现在看来，这一梦想只有等他的重孙辈去实现了。
2. 用连接代词引导的主语从句
对于以连接代词（副词）引导的主语从句，可以使用形式主语代主语从句，也可直接在句首使用主语从句。
例如：
Whether they would support us was a problem.
他们是否会支持我们还是一个问题。
It was a problem whether they would support us.
他们是否会支持我们还是一个问题。
3. 关系代词型what引导的主语从句
对关系代词型what引导的主语从句，通常直接将主语从句放在句首。
例如：
What we need is money.
我们需要的是钱。
What I want to know is this.
我想知道的就是这一点。
有时也可用形式主语
如：It is clear enough what he meant.
他是什么意思很清楚。
4. 如果句子是疑问句
则必须用带形式主语it的结构：
例如：
Is it true that he is the girl's father?
他是那女孩的父亲，是真的吗？
How is it that you are late again?
你怎么又迟到了？

连词that的省略问题

引导主语从句的连词that有时可省有时不能省，其原则是：若that引导的主语从句直接位于句首，则that不能省略；若that引导的主语从句位于句末，而在句首使用了形式主语it，则that可以省略：
例如：
That you didn't go to the talk was a pity.
很遗憾你没去听报告。（that不可省）
It was a pity (that) you didn't go to the talk.
很遗憾你没去听报告。（that可省）