I want to disagree slightly with previous responses. #1 can properly be used only to refer to learning to play, or perhaps becoming a professional player. It is ungrammatical otherwise. Eg: "I told my mother I wanted to become a guitar player, but she said, 'I want you to play piano.'" #2 can be used the same way, but can also be used to refer to a specific piano. Eg: if there is a violin in the room and someone asks me to play a song, imight say, "which instrument?" He replies, "please play the piano" #3 is used when we dont have a piano, and any piano will do. Eg "go find a piano for the party." But a native speaker would almost never say "play a piano." And really even "play piano" is pretty rare. Most people (at least in the USA) say "play the piano"

Hi Emu, In (1) you are giving more of a direct command. You are saying that you want someone to play the piano, insinuating that there is a piano present in the setting, and that you would like that someone to play it in that moment. In (2), it is similar to (1). You can take the statement the same way, where you presume there is a piano in the setting, and you'd like someone to play it for you in that moment. You can also take this situation as a more general statement, like when a parent tells you that they'd like you to start taking piano lessons, because they want you to learn to play the piano. In that case, (2) makes sense too. Because you say "the piano" instead of "piano" you can direct it to a specific piano in the setting. In (3), I don't really hear it that often. The way I picture it in my mind is that you are in an instruments shop, and you are choosing a piano to buy, and therefore you need to play one to see the difference. It is a more general statement, and although it makes sense, (1) and (2) are more common. Hope this helps. Let me know if there's more clarification needed!

I would say 1) is when you are talking to someone and express a desire for them to play. There may not be an actual piano present, you are saying you want them to play, in general. Or it could be meaning - out of a choice of instruments, piano is the one I want you to play. 2) This is very similar to 1, and in fact it is just a manner of speech whether you say the piano or piano, although 1) sounds more cultured to me, as coming from one who is familiar with musical instruments. 3) Sounds odd. You would say this if, for example you were directing a mime and telling someone, now I want you to play a piano - meaning move your hands as if playing. A piano is too general to be used in a specific conversation about someone's musical career or choices. If there were a row of pianos and you were directing them to play on any of the pianos present, then that also would be appropriate. But it would rarely come up in a grammatically correct sentence.