1. Watching TV is fun.
2. To watch TV is fun.
3. It is fun watching TV.
4. It is fun to watch TV.
在目前的國中英語，學生只學1, 2, 4的用法。
3 有時候還被當成錯誤的誘答選項。
可是外國人認為3也是正確的，反而2很不自然!
All are grammatically sound, but #2 is unidiomatic.
#4 is probably the most natural.