playing是动名词,所以playing the piano 可以作主语.play 是个动词,不具有名词性质.
比如说,可以这样造句,playing the piano is interesting.此处不可以用play
I can play the piano.这里则不能用playing
但是还有一种情况是根据前面的动词.I like playing the piano.或者i like to play the piano.
都是正确的.因为like既可以加doing 又可以to do