When do we put "the" before musical instruments?
E.g:She plays the violin.
She's holding a violin.
Why we used the in the first example but not in the second?

Thanks.

Question

When do we put "the" before musical instruments?
E.g:She plays the violin.
      She's holding a violin.
Why we used the in the first example but not in the second?

Thanks.

Answer

It is just the way we say things.
"She plays violin." = "She plays the violin." = "The instrument she plays is a violin."
It means that she plays an instrument and the instrument is a violin and she plays it in general.  It could be at any time.

"She plays a violin." could also be used to mean she often plays violin.  However, it is used more to refer to a particular place and time.
"She plays a violin in the orchestra."  = "She plays violin in the orchestra."
You also might be referring to a particular time.
"Last night, she played a violin."  Note that there is no hard rule.  We could have said "Last night, she played the violin." - without talking about a particular violin.

In slang "She plays a mean violin" = "She plays the violin very well."
Hope this isn't too confusing.

Answer

This is a stock phrase - to play the violin, to play the euphonium.

If we use 'a', the violin could be any violin (indefinite).