有时为避免句子头重脚轻,常用形式主语it代替主语从句作形式主语放于句首,而把主语从句置于句末。主语从句后的谓语动词一般用单数形式。常用句型如下: 
(1) It + be + 名词 + that从句
It is common knowledge that      是常识
It is a surprise that      令人惊奇的是 
It is a fact that      事实是 
(2)It + be + 形容词 + that从句
It is necessary that      有必要 
It is important that      重要的是 
It is obvious that      很明显 
(3)It + be + 动词的过去分词 + that从句 
It is believed that      人们相信 
It is known to all that      众所周知 
It has been decided that      已决定 
(4) It + 不及物动词 + that 从句 
It appears that      似乎 
It happens that      碰巧
It occurred to me that     我突然想起
另注意:在主语从句中用来表示惊奇、不相信、惋惜、理应如此等语气时,谓语动词要用虚拟语气“(should) +do”,常用的句型有:
It is necessary (important, natural, strange, etc.) that
It is suggested (requested, proposed, desired, etc.) that