I think its fun

应该是 I think it's fun.

fun 这个词既可以是名词，也可以是形容词，fun前面如果加了形容词，例如great，就可以作为名词来用，例如：I think it's great fun. 如果fun前面没有形容词，则可以把fun理解为形容词， 例如：I think it's fun. 但不能说：I think its fun. 楼主给的例句可以合并为一句：She thinks it's fun to play sports.

it's funny吧
系表结构
fun是名词，
funny才是形容词
its fun单独用在thinks后面不行的，因为think后面要跟的是一个完整的结构，不能只是一个名词。

原句错了，就是it's fun

I think it's funny。
所有不用it's

不懂 好像是第三人称的原因吧