当主语过于长的时候，为了方便表达，我们会使用形式主语占住这个“位置”，然后在句子后面以其他形式把实际的主语给显示出来。
占住这个位置常用it来表示，it就称是形式主语。
it作形式主语代替主语从句时，形式主语it没有具体的含义，只是帮助把真正的主语移到句子后面去，使句子显得平稳一些。此时，主语从句可以用从属连词that引导，也可以用连接代词或连接副词引导。
it作形式主语，把真正的主语从句移至后面的常见句型有以下五种：
1，It+be+形容词+主语从句
此类形容词有strange,natural,surprising,obvious，true，fortunate，wonderful，funny，possible，impossible，likely，clear，unusual，certain，important，necessary 等。
It is doubtful whether he will be able to come.
他能否来还很难说。
It is important that we should learn from each other and help each other.我们应当互相学习，互相帮助，这是很重要的。
2，It+be+ed分词+主语 从句
此类动词的过去分词有said,reported,announced,decided,expected,hoped,known,believed,decided,discussed 等。
It is said that nothing has been done about the flood.
据说至今对于洪灾没有采取任何措施。
It is known that he is one of the best employees in our company.
大家都知道他是我们公司最好的员工之一。
3， It+be+名词+主语从句
符合这种句型的名词有pity,shame,fact,honor,news,wonder,surprise,mystery 等。
It is a pity that the rain spoiled our holiday.
真可惜，这场雨把我们的假期给搅了。
It is a wonder that more people weren't hurt in the earthquake.
奇怪的是地震中没有更多的人受到伤害。
4，It+不及物东西+主语从句
此类不及物动词有 appear,seem,happen,turn out,strike,occur等。
It happened that I went out that day.
恰好那天我出去了。
It suddenly occurred to me that I met him before.
我忽然想起我之前遇到过他。
5，特殊句式
It doesn't matter+连接代词或副词引导的从句
It doesn't matter whether she comes or not.
她来还是不来都没关系。
lt doesn't make much/any difference+连接代词或副词引导的从句
It doesn't make much diference whether you come back today.
你今天回不回来，关系不大。
