So this girl I know has a horrible boyfriend that everyone, including me, hates. She seems nice and started to talk to me for a while. I got to know her, and I liked her, and it seemed she was into me as well. Recently she stops talking to me, and starts to act like she doesn't even know me at time. Then she goes and tells one of her friends that she is really into me. No idea how to approach this situation.

You leave her alone. While you may not like her boyfriend, he is her boyfriend. Their relationship is something they have to work for themselves.

If she does break up with him, don't immediately think about trying to date her. She will need time to heal and let go of the past relationship. Sliding from one relationship into another one ends up causing issues in the new one.

Accept that she may be backing off because she doesn't want to be tempted to cheat on her boyfriend or she may feel like having feelings for you is already cheating on him.

Live your life and allow her to live hers. Don't become her crutch for dealing with her current relationship. It isn't fair to you.

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How old are you two? If she's ignoring you, then there's little you can do. You can't force the issue with her.

You have to talk to her about it.

you have to talk to her about it.

To what end?

She has a boyfriend. Like him or not, that means that she's not available. Talking won't help. She has to decide what she wants, and seeing as she's still with her boyfriend she's made the choice that, for now, he's what she wants.