If someone sins against me do I have to talk about it with them to forgive them?

​The simple answer is no you don’t. While Jesus is clear in Matthew 18 that Christians need to forgive people, the Bible doesn’t say you have to talk to someone to forgive them. This is because forgiveness is primarily something which is “from your heart” (Matthew 18: 35, ESV). If forgiveness was about saying to someone “I forgive you”, it would be essential to have a conversation with them. However, as forgiveness is primarily something which comes from our hearts, a conversation is not always needed. Indeed, there may be situations in which it is not helpful, constructive or possibly even safe to approach someone about their sin against you. In this case, it is still important that we do forgive them “from the heart”.
That being said, the normal way of dealing with other Christian’s sin which Jesus advocates is to “go privately and point out the offence” (Matt 18:15, NLT). This means generally being prepared to have a chat with someone. This isn’t just about you forgiving them, this is also loving them and leading them by helping them to see where there is sin in their life. They might not even realise they messed up! The best way to do this would be to take the lead from Jesus; don’t go preparing to condemn them, but aiming to gently serve them by letting them know what they’ve done wrong. Check out Matthew 18:15-20 to think more about this one. If you’re English like me, this doesn’t come naturally!