1. Watching TV is fun.
2. To watch TV is fun.
3. It is fun watching TV.
4. It is fun to watch TV.
在目前的国中英语，学生只学1, 2, 4的用法。
3 有时候还被当成错误的诱答选项。
可是外国人认为3也是正确的，反而2很不自然!
All are grammatically sound, but #2 is unidiomatic.
#4 is probably the most natural.

当然学最natural的比较好囉！