it作形式主語

有時為了考慮句子平衡，通常在主語從句處使用形式主語it，而將真正的主語從句移至句末。這分四種情況：
1. 對于以連詞that引導的主語從句，通常用形式主語it替代主語從句；
2. 對于以連接代詞（副詞）引導的主語從句，可以使用形式主語代主語從句，也可直接在句首使用主語從句；
3. 對關系代詞型what引導的主語從句，通常直接將主語從句放在句首；
4. 如果句子是疑問句，則必須用帶形式主語it的結構。

四種基本句型

1. 連詞that引導的主語從句，通常用形式主語it替代主語從句。
(1) It is +名詞+從句
It is a fact that... 事實是……；
It is good news that... 是好消息；
It is a question that... 是個問題；
It is common knowledge that... 是常識
類似的名詞還有：a pity, a wonder, a good thing, no wonder, surprise等。
it is no surprise 毫不驚奇地
例如：
It is a mystery to me how it all happened.
這是怎么發生的，對我來說就是一個謎。
It is common knowledge that the whale is not a fish.
鯨魚不是魚，這是一個常識。
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.
中國在很多尖端領域，如大飛機制造，高鐵、干細胞、航天航空研究等領域取得飛速發展一點都不驚訝。
(2) It is +形容詞+從句
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……
類似的形容詞還有：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.
我們很懷疑她是否能來。
It is essential that he should be here by the weekend.
周末他務必要來這里。
It seems obvious that we can not go on like this.
很明顯，我們不能再這樣繼續下去。
It is necessary that you (should) master the computer.
你必須要掌握電腦技術。
(3) It is +過去分詞+從句
It is said that... 據說……；
It is reported that... 據報道……；
It has been proved that... 已證明……；
It must be proved that... 必須指出……
類似的過去分詞還有：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.
過去人們說女性必須做兩倍于男性的工作才能獲得一半于男性的好評。
It has not been made clear when the new road is to be opened to traffic.
目前還不清楚，該路段何時可通行使用。
(4) 其他
It seems (happened / appears / doesn't matter / makes no difference / occurred...) that... 主語從句不可提前
例如：
It makes no difference whether he will attend the meeting or not.
他是否出席會議并沒有什么不同。
For now, it appears that the dream might have to wait for his great-grandchildren.
從現在看來，這一夢想只有等他的重孫輩去實現了。
2. 用連接代詞引導的主語從句
對于以連接代詞（副詞）引導的主語從句，可以使用形式主語代主語從句，也可直接在句首使用主語從句。
例如：
Whether they would support us was a problem.
他們是否會支持我們還是一個問題。
It was a problem whether they would support us.
他們是否會支持我們還是一個問題。
3. 關系代詞型what引導的主語從句
對關系代詞型what引導的主語從句，通常直接將主語從句放在句首。
例如：
What we need is money.
我們需要的是錢。
What I want to know is this.
我想知道的就是這一點。
有時也可用形式主語
如：It is clear enough what he meant.
他是什么意思很清楚。
4. 如果句子是疑問句
則必須用帶形式主語it的結構：
例如：
Is it true that he is the girl's father?
他是那女孩的父親，是真的嗎？
How is it that you are late again?
你怎么又遲到了？

連詞that的省略問題

引導主語從句的連詞that有時可省有時不能省，其原則是：若that引導的主語從句直接位于句首，則that不能省略；若that引導的主語從句位于句末，而在句首使用了形式主語it，則that可以省略：
例如：
That you didn't go to the talk was a pity.
很遺憾你沒去聽報告。（that不可省）
It was a pity (that) you didn't go to the talk.
很遺憾你沒去聽報告。（that可省）