Method 3 of Think about your needs. Before you become too attached or involved with the girl in a relationship, ask yourself what you're hoping to achieve. Do you want to simply become good friends because you think you have a lot in common. Or are you looking for a romantic relationship. Since the girl is in a relationship, you need to figure out:  X Research source If you're willing to wait for her current relationship to end If you can move on emotionally if she remains in her current relationship If you'd be fine with looking for a different girl who's available to date. Decide if you should ask her out.
In fact, it was the most common response from the guys I informally surveyed. It's hard to come right out and say it, and sure, it probably doesn't feel great to hear, but it's ultimately the best way.
A standard, slightly gentler line. Think, "You're a nice guy, but I'm just not into you in that way.