Trust and communication
Healthy relationships are built on trust and openness. If your child tells you their boyfriend or girlfriend is often suspicious about what they're getting up to, or shows a lot of jealousy, it might be helpful to talk to them (in a general way) about what makes for good relationships and bad. What they've noticed among their friends, for example.
What do they think about the honesty of other relationships they've seen, including among their family and your friends? (Note to self: the more trusting and open you are able to be in your own relationships, the better template they'll have). If you are dishonest and evasive with them - not that you would be - they're more likely to expect that from others.
Abusive individuals usually attempt to control their partners’ lives. This may mean dictating what clothes they wear, what they think and say, and the people they see. They need to separate their targets from other people so they become isolated and dependent. This is, of course, not to be confused with being besotted with a new person and spending all their time thinking, talking about and being with them.
Abusers destroy the self-esteem and confidence of their victims. They may say hurtful things about appearance, intelligence or fashion sense. Young people are insecure enough already, without this kind of 'help'. Support your child's self-esteem by praising them for things they do well, teaching them to be realistic about body image and airbrushed celebrities and stressing that no one who cares about them will try to tear them down.
The best relationships often encourage us to try new things, like a new sport or an adventurous food. But young people in relationships are sometimes pressurized to do things they don't want to do, including engage in sexual activity before they're ready, or share naked photos online. Make sure your child understands that they have the right to say no and that anyone who cares about them should respect that. And warn them, on the photo front, that once it's out there, you don't control it any more.
It’s all right to argue, but there are better and worse ways of doing it. It's even fine to be angry - if the emotion is justified and honestly expressed - but you need to be able to control your anger. Violence in realtionships is horrible and no one should have to accept it. Make sure your child knows that there are healthy and really really unhealthy ways to disagree. And make sure they know they can always come to you for support.