I'm convinced our son (who's 31) is about to make a big mistake by marrying a woman who's very selfish and has been married several times before, but he won't listen. I suppose we ought to leave him alone, but how can we? We don't want to see him get hurt. — E.McC.
No parent likes to watch their children go down a wrong road, and it's natural to want to intervene and try to help them realize what they're doing wrong. In fact, this is part of our responsibility as parents.
But when our children become adults, we no longer have control over them -- whether we like it or not. We can warn them (and we should), and we can try to help them see what is ahead for them (and we should) -- but ultimately they must make their own decisions, and we even must give them the freedom to make their own mistakes. Remember, too, that you don't know your son's future wife as well as he does; people can change, and perhaps she has changed and they'll find lasting happiness together.
If your son does marry this woman, I hope you'll welcome her into your family -- in spite of your concerns. On the other hand, if his decision turns out to be wrong, let him know you still love him. I'm sure the father of the prodigal son in Jesus' parable knew his son might waste his inheritance and almost destroy his life -- but he still welcomed him home: "This son of mine was dead and is alive again; he was lost and is found" (Luke 15:24).
Pray for your son, that God will guide him (even when he isn't aware of it). Most of all, make Christ the center of your life, and urge your son to do so also.