Should you tell a girl how you feel if she has a boyfriend?

yes,I think you should because if she breaks up with him you will be a number one pick. If she knows you like her then she may want to go out with you if she has broken up with her old boyfriend.

No, you should respect the fact that she has a boy friend and is taken. Flirting with a taken girl is simply wrong and could get you in trouble maybe even hurt.