Specific?
Definite or indefinite.
“He is the boy playing the guitar.” = there is only ONE boy playing the guitar. This boy talked about is THAT boy. That definite boy.
“He is a boy playing the guitar.” - there is one boy, or maybe are many boys playing guitar.
He is one of them. He is an indefinite boy of several.
.
Which boy do you mean?
He is the one playing the guitar, while the others sing.
.
Which boy do you mean?
He is a boy playing the guitar, but not the ones on the end; the one in the middle.
He is one of the boys playing the guitar, but not the ones on the end; the one in the middle.