英语语法中的形式主语是什么概念,请举例说明

比如
It is said that she was lost in the forest monday.
It就是形式主语

如:It is a good idea to play football this afternoon.中的It即为形式主语．就是相当于主语，充当一个样子，占了真正主语的位子．真正的主语是后面的to play football ，动名词短语做主语．与其相应的还有形式宾语．

当主语、宾语是很长的从句、不定式、动名词时常用“形式主语”和“形式宾语”来替代,而真正的主语、宾语置于句末.
1.用“形式主语”的情况下,说明真正的主语（从句、不定式、动名词）较长, 用“形式主语”是比较符合英语习惯的.不用也可,但句子显得头重脚轻.如：
That he won the game is true.
不如It is true that he won the game .
2.在要用“形式宾语”的情况下,“形式宾语”是必须要用的,如：
I find it hard to get along with her.
不可把to get along with her放在find 和 hard 之间.