1、It is fun to do......
指具体做什么很有趣，指具体动作。当“逗乐的，有趣的，使人开心的”时。
如：It is fun to read books（读书是有趣的）
2、It is fun doing......
指一种爱好，觉得有趣。fun当名词，当“享乐，乐趣，快乐，享乐的事”时。
It is fun playing the piano（弹钢琴很有趣）
扩展资料：
it's fun后面接不定式或动名词均可，意思基本相同，表示做某事很有趣。
如：It’s great fun sailing a boat.= It’s great fun to sail a boat. 扬帆驾舟十分有趣。
have fun(in)doing sth 表示做某事很开心。
在一些特殊的句型中，常用it作形式主语，把动名词后置，如：
It's no use crying over spilt milk.
It's pointless arguing with him.
It's tiring looking after my nephews.
It's no good helping him.
It's fun learning English.
