It is great fun playing with him分析

It在句中作形式主语,真正的主语是动名词短语playing with him.
这个句子也可以这样说,Playing with him is great fun.
但英语中有时习惯用it做形式主语,这样句子显得不那么头重脚轻.
例如,
It's so nice to meet you.
It's kind of you to say so.
等等.
希望能帮到你哦!