It is great fun ___(play)the piano .
问题描述：
It is great fun ___(play)the piano .
答
It is great fun to play the piano.
答
It is great fun to play the piano .
弹钢琴很愉快。
It's +adj./n. to do sth.
答
It is a great fun (to play) the piano.
弹钢琴是一大乐趣.
to play the piano是动词不定式作主语,it是形式主语.