Find out if they have a girlfriend/boyfriend. If they don't, you are free to ask but if they do have a boyfriend/girlfriend, you should just be friends for a little bit. When they break up with who they were going out with, give them time to heal then go for the move.
only if you like them back
Try to get them to like you back or move on
I hope you like me back ;)
Then you should consider yourself fortunate.
Leave them alone together. Find someone else to like.
You should find someone who likes you for you and if they don't then they are not good enough for you.
well your just a rebound
Yes, if you like them back and if you are both ready for a relationship.
If you like them flirt back :) or if you don't like them just ignor them
You might as well go for it before someone else does.
Go back to the person and be like "Wow you've really changed. Do you think you like me anymore?"
Try flirting with him/her, but not in front of their girlfriend/boyfriend. But then again, be friendly, you may want to be FRIENDS with them first.