You can compliment her but dont make it look like you are perfectly OK hitting on a girl who has a boyfriend; let her start to see that theres something special about you. Even if she is entertaining the idea of being with you, nothing will put it out of her mind faster than if you blatantly try to hook up with her.

Is it bad to flirt with a girl who has a boyfriend?

If youre flirting because shes cute and fun and you find flirting in general fun, thats fine. Shes the one in a relationship, and she knows what is and isnt ok in that relationship. Therefore, if shes flirting with you, you can probably assume that a little harmless flirting is ok for her and her boyfriend.

What to do if a girl I like has a boyfriend?

If youre interested in a girl who already has a boyfriend, you need to show her that you like her and that you would make a good boyfriend. Try telling her how you feel. Then, its up to her to make the next move and break up with her boyfriend so you can have a romantic relationship with her.