:when you want to say "playing piano or other music instrument" you don't use "the".
however, it could change depending on the situation. for instance, general concept, you don't use "the" at all just like you said. but if it is a specific time, you need to use the. see below example.
1) I play piano
2) I am playing the piano
3) I played the piano
all three above is correct sentense. as you know you use "the" when it is indicating specific thing. so when you are playing piano right now, people must know which piano you are playing and you must know which piano you are plying right now. understand?
also past sentsence, if you want to tell other people, "i played piano" it does not make a sense because it is past action so you must know which piano you used. that's why you have to use "the" when you want to use the past sentence.


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