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.