If your teen has a boy/girlfriend, you must have figured out by now that it is the kiss of death to actually say what you really think about this person. It is never the right thing. Either you can't stand the kid because he/she is not nice enough, not smart enough, not polite enough, dresses in a way that makes you crazy, is a suspected drug/alcohol/sex fiend, likes to party too much, gets bad grades, and has a family right out of the sopranos, or he/she is so nice, so polite, so smart, motivated and responsible, and has a family right out of the Cosby Show. Either way, your opinion of this person and your sharing of it is likely to push your teen in the opposite direction of your actual desire to either break them up or commit them to each other for the the rest of high school so you don't have to go through this parade of boy/girlfriends for the next 4 years.
This boy/girlfriend dilemma is a complicated one for parents. When you see your teen with someone you feel can be a bad influence on them, pulling them into situations you think will be unsafe, emotionally unhealthy, and that potentially could have a detrimental affect on their future, your mama/papa bear claws come out. You share your "insights" about this person with your teen, expecting they will listen, learn, respect your opinion and do the right thing....break up with this bum! However because your teen is now biologically and emotionally driven to think just the opposite of you, in a show of "well I'm not you", are now more motivated than ever to dig their own claws in to their new love as a show of independence. One of the major tasks of adolescence is what we call "separation". This is literally developing the ability to stand on their own two feet, in preparation for their future life as an adult. There are some things they are willing to admit you know more about, albeit reluctantly, like academic issues, but their friends are completely off-limits to you and your opinions. This is an area of their life they feel is their birthright and expertise. Be damned with what my parents think?
Here is what you can't and should never do. Never, ever say to your teen;" I don't want you to, you are not allowed to go out with that person! Remember Romeo and Juliet, this is a set up for lying and sneaking behavior. The bottom line is you have no control over who they see. Unless you lock them in their room and home school them (only kidding, don't run out and buy a teaching manual), you no longer have control over their play dates. They see this person at school, after school and on weekends. Again unless you have a nanny cam attached to their person, there is no way of knowing when and if they are hanging around with this object of your disaffection. Also directly giving your assessment of this person to your teen can only serve to shut off communication rather than keep it open. If they know you already can't stand this person, why would they come to you if they actually need your advice or consolation. "I told you so's " do not contribute to open communication.
Here is what you can do: You can use "I Get It" starters. Rather than directly expressing opinion start with something like this: "Honey, I can see why you like Romeo, he's a cutie, and funny, and a little bit wild. I get it, he's a fun guy. Tell me what you like about him. " Give her the opportunity to tell you about some of the things about this person you may not know. Maybe his standoffishness is shyness for example. When you ask open ended questions, you are showing real interest in the people that interest her, and also trying to find out what this relationship really means to her. Try not to be judgemental or critical, this will not serve you well. After she has shared something about Romeo, you can say " I am happy that you are hanging with someone that is making you happy, but what do you think I am worried about in your relationship with him?" Your daughter/son knows what you are worried about, but if they say it rather than you saying it, they will be less likely to get defensive and evasive. After they say all the things you would have said, you can say:"Yes I do worry about those things, what do you think you can do to make me feel OK about them, so that we don't need to fight so much about this? I love you, and I just want you to be safe, and do what you need to do to get on with your life."
Ultimately your kids want your approval. However if you push them into the corner by trying to control their natural impulses to stand on their own two feet they will let you know in a clear and direct manner to "stay out of my life". So the work here is to help them articulate what relationships mean to them. Relationships in adolescence are a training ground for relationships in adulthood, and experience in all kinds of relationships will serve them well in the future. They need your counsel not your control.