Use 'the' with instruments (play the piano) and usually no article with sports (play tennis).
play + the + instrument
Do not drop 'the' with instruments in standard usage.
Do you play the guitar?
💡 In standard English, musical instruments commonly take 'the' (play the piano). Sports usually take zero article when speaking generally (play tennis). British vs US note: the article pattern is the same; the main differences are vocabulary choices (e.g., BrE football vs AmE soccer).
✓She plays the piano.
✗She plays piano.
Instruments typically take 'the' in standard usage.
✓He plays tennis.
✗He plays the tennis. (general)
Sports usually take zero article when speaking generally.
✓They bought a guitar. Now she is learning the guitar.
You can use 'a' to introduce the instrument, then 'the' for reference back or as the generic instrument type in this context.