It sucks when you have a crush on someone who is already taken. The constant disappointment can take a toll on your emotions. You know what you want, but there's always that barrier between the two of you that has the potential to ruin everything: the significant other.
Here's my general rules for what to do in these situations. I'm going to be speaking of the situation of a girl crushing on a guy who already has a girlfriend, simply because I have been on both sides of this... I have had crushes on people who were already taken, and yet at the same time I have a girl or two who are still chasing after my boyfriend (and they piss me off to no end). This same advice can be taken by a guy crushing on a taken girl... just flip all the genders. Or if you're a guy crushing on a guy, or a girl crushing on a girl... just substitute the proper pronouns. I have faith that you can.
1. Do not call him "babe", "hun", "sweetie" or any other pet names. Those are reserved for his girlfriend and her only. If she finds out you call him any of these things (whether it be in texts, over AIM, or even to his face), she WILL be pissed. It makes it obvious that you want something more than friendship. This makes her suspicious of your relationship with him.
2. You may think it sounds like fun to fuck with his girlfriend and make her think there really is something going on between you and her bf. This is a BAD idea. When she gets mad at the bf, more than likely she is going to want him to STOP SPEAKING TO YOU. This means that you lose out. Not exactly what you were going for. You may think that he would never let a girl control him like that... but the reality is that after fighting about it for a week, he'll do anything just to make her shut up. If it comes down to a choice between you and her... I know it sucks, but he'll probably choose her every time. If you were more important and he wanted to choose you every time, he'd be with you, and clearly he's not.
3. When the two of them get into fights, give advice IF ASKED, but never let your bias show. If every time they get into a stupid little fight you tell him to break it off, he's going to learn that you give shitty advice and he'll never listen to anything you have to say on the subject. When giving advice, try to give it from the same standpoint you would if giving advice to your brother. Do not give advice just because you think it will be good for you in the long run. If he asks you, he values your opinion. Keep it that way.
4. Don't be overly flirty with him. OK: laughing at his stupid jokes, having inside jokes with him. NOT OKAY: playing with his hair, hugging him (except for a hug good-bye), holding his hand, or any other unnecessay physical contact. Whether his girlfriend is there, or you're hanging out with other friends, or even if you're alone with him, it doesn't matter. No matter who the audience is, he will know exactly what you are doing. He may or may not like the attention you are giving him. If he doesn't like it, then he will make a mental note in his head that you are looking for more than friendship, which will detrimentally affect your friendship in the long run. If he is interested... even if he WERE to break up with his girlfriend to be with you... do you really want the type of guy that will fall victim to any flirtatious girl that crosses his path? Furthermore, this situation will cause a bad relationship between him and his new ex, which will make him unhappy (and trust me, you don't want him upset), and turn the ex into a vampire-esque cannibal that wants to suck your blood dry (you do not need this complication either).
5. Don't complain to him about how him having a girlfriend sucks. Even if he blatantly knows that you are crushing on him. He OBVIOUSLY won't agree with you. If he did, he WOULD'T HAVE A GIRLFRIEND. This is a stupid pity-seeking move. He will think you are pathetic in this attempt, which gets you nowhere.
6. Hanging out with him is fine. Getting mad at him when he's HAD plans with his girlfriend and you want to make last minute plans, on the other hand... You have to accept the fact that in most cases she will come first. It IS ok to let him know how upset you are if he cancels plans with you to hang out with his girlfriend. Don't automatically turn it into an argument - let him know that you are hurt by it and say that he owes it to you to make a new set of plans for some time soon.
In general your plan should be to:
Hide your feelings (it's ok to tell him, but not okay to say it more than once, or to show it physically)
Be his friend first and foremost. That's what he wants out of you right now
Let things play out between him and his girlfriend, without interfering
Keeping to these allows you the greatest chance of EVENTUALLY getting with him (although I make no guarantees). It also gives you the best quality of friendship in the in between time.
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