play the piano加the是英语用法规定的，就像汉语里的一些固定用法一样。play the piano意思是弹钢琴。play the piano的例句：She'd love to be able to play the piano（她很希望能弹钢琴）。

play the piano的例句：He learnt to play the piano at an early age（他早年就学会了弹钢琴）；I never expected that you could play the piano（看不出来，你还会弹钢琴）。