I play the piano. I play piano.
I used to think that we need "the", but now I often see the phrase without it.
What is the difference?
I have also heard that when someone plays clarinet in the orchestra, then you say "I play clarinet" without "the". Is this true?
There is nothing wrong with saying "I play piano" ("I play clarinet," "I play saxophone," etc.) instead of saying "I play the piano" ("I play the clarinet," "I play the saxophone," etc.).
If the British have an aversion to the version without "the," that is not my problem. I have an aversion to their omitting "the" in "He is in university," "He is in hospital," both of which are ungrammatical to the American ear.
I played (the) clarinet for many years. For most of those years, I played (the) clarinet in orchestras. Sometimes, when I speak of this, I use "the" before clarinet. Other times, I don't.
It has never made the slightest difference to me or to any English speaker with whom I have interacted whether "the" is used before the name of the instrument being referred to in such sentences.
Grammatically, however, it is clear that the usage with "the" is a generic usage, comparable to "the" in sentences like "The lion is a ferocious animal." It is also perhaps more traditional to use "the," even though the version without it is fine.
While I have documentation as to the generic use of "the" in "I play the piano," etc., I have no documentation as to the grammatical nature of the version without "the" ("I play piano," "I play clarinet").
I would say that, in the "the"-less version, the name of the instrument is being used as a noncount noun. It is being pressed into noncount usage, just as we sometimes press noncount nouns into count usage (e.g., "poetries").
There are other phrases in which the names of instruments are sometimes used as if they were noncount nouns. For example, we can say, "I am hearing too much piano in this passage. Can you please play softer?"