名词性从句做主语时，在结构形式上会给人一种头重脚轻的感觉(如上述各例句），为避免 这种句式结构的不平衡，常借用代词it来代替此名词性从句，而将此从句进行后置，这时的it 就是形式主语(假主语），真正的主语是后置的主语从句（真主语）。
It is a miracle that he survived the earthquake.
It makes no difference whether he agrees to the plan or not.
It has never been explained why university students seem to enjoy practical jokes more than anyone else.
a.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
It is strange that... 奇怪的是
It is likely that... 可能的是
It is true that... 诚然
It is quite evident that a storm is coming.
It is clear to me that he is not a liar but a truth seeker
b. It + be + -ed 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
It must be pointed out that...必须指出的是
It is to be discussed that...有待讨论的是
It is said that...据说
It has been decided that the book be revised.
c. 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 pity that...遗憾的是
It is no wonder that...不足为奇的是
It is a mystery that...让人神秘不解的是
It is a little known fact that there were already half a million Germans in America at the time of the American Revolution.
d.It +不及物动词+ that-从句
It happens that...碰巧......
It occurred to me that...我突然想起......
It turned out that...结果是......
It seems/appears that...好像......
It was not until he got to an ATM that it occurred to me that he had left his card at home.
It does not matter whether they turn up or not.