在乐器前,应该加上定冠词the,即the piano.而 play the piano 是固定用法,表示弹钢琴.
CAN YOU PLAYING PIANO?这句话从语法上看,情态动词can后面应该用动词原型,即play .
此句可以改成“Can you play the piano?”或者“Are you playing the piano?”
定冠词the地用法可以参见下面的参考链接.