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Don't mislead with your body language. Handle a public proposal. Stay positive but honest. Respond to their unhappiness and confusion. Explain your feelings. Consider the way forward. Avoid making conditional agreements.
Try not to smile when proposed to, or at least not to beam and look excited. If your mate has gone as far as to pop the question, they are assuming you will say yes, and your smiling will only confirm their hopefulness, which will make the shock of your rejection greater. Look in your partner's eyes tenderly, put your hand over theirs, and give your response in a gentle voice. If you are in public when proposed to, ask your suitor to stand up (if necessary), and give them a gentle hug. This is a way to acknowledge that you're touched by your partner's gesture, without saying yes. Hopefully, it is enough to make anyone who's watching lose interest and go back to doing whatever they were doing, which will help ease any embarrassment for your mate.  If people are still watching, take your partner by the hand and quietly ask to go somewhere more private. Never say yes just to get out of the situation so you can give your real response later. This will just make the later rejection more painful. This guide assumes you like the person proposing; if your ex from three years ago leaps out of the bushes holding a ring, "say no and get out of there" is all the advice you need. But if the suitor has reason to be hopeful, it's best to break the news gently, without being misleading. Say something appreciative, but keep it short so you can get to your response:  "That's such a moving request. I need time to think about it; it's not something I'd feel right saying yes to straight away. It's a bit of a surprise for me - would you mind if I take some time to think it over?" "Thanks, that's incredibly wonderful of you but I have so many questions that I've yet to ask you that I'm not sure of our future compatibility yet. Maybe this is a good time for us to sit down and talk about what spending a life together would look like." "I love you for asking that, but I don't plan on getting married (for a while / ever). We could move in together instead?" "I'm honored that you've proposed to me. You're a kind, caring (man / woman). I wish I could say otherwise, but I can't see our marriage working out, and I have to say no." It's likely that the person proposing has put in a lot of effort to asking, thinking through the depths of why they want to spend the rest of their life with you. Letting down their unhappiness gently isn't going to be easy, but there are ways you can help the process:   Give each other space if either of you feels angry or too upset to talk. Let your partner know you'll be contacting them soon (preferably the same day or next morning), but give them time to think until then. Suggest the two of you do something you both love doing. If your partner wants affection from you, spend the rest of the day together doing something that keeps your occupied. This will serve as a distraction and it will help the other person realize that you still care. In private, and once neither of you are actively upset at each other, explain at length what the situation is. Emphasize that your relationship still matters a great deal to you. Describe exactly why you're not prepared to accept. Don't let the other person think it's because they're not good enough. If you have doubts about the relationship itself, not just marriage, be honest about this. It may not be the best time to air your grievances, but let your partner know that there are problems to discuss once you've both calmed down. Things can get really awkward from this point, or they can pick up as usual with good, caring approaches to continuing to nurture your love and relationship. If the proposer is able to accept your position (whether that's an alternative to marriage or delaying the decision for now), then the relationship will likely remain strong and forge on. On the other hand, if this rejection opens a rift in your relationship and causes suspicion, anger, resentment, or discomfort, it may be time to reassess the purpose of staying together. Unless it's clear that the relationship has ended, take a few weeks to sort through the post-proposal emotions before making a drastic decision. Telling your partner that you'll say "yes, if..." is not about love or reasonable compromise. You may end up in the same situation a year from now, full of resentment that you backed yourself in a corner. Instead, ask for more time to work out what it is that makes you want to reply conditionally. Most likely your real answer is "no," and you should only alter this answer if you have a genuine change of heart.