Without looking at the dictionary, I'll stick my neck out to say that "abstain," except in the sense of voting baqarah mentioned above, is not used to describe a single occurrence. In other words, you would say that when your sister-in-law displayed her usual lack of tact in criticizing your new dress, you refrained (not abstained) from telling her what you thought of her, in the interest of preserving family peace. Also, "abstain" is used more often to describe a long-term commitment to avoiding something, as in sex (also mentioned by baqarah), drinking alcohol, etc. Thus, "I promised my mother I would abstain from drinking hard liquor, and I've kept my word, even though she's been dead for twenty years," but, "I try to refrain from drinking if I'm already feeling tired, or if I know I'll have to get up early the next morning."
[not a teacher]