There is no way to know who you are writing to if you just reply in a thread without quoting anyone. The quote feature is on the forum, so you can use it, and people know who you are talking to. Let's say you want to reply to my post, just click on quote and write your post, and click onthe  post button. It's that easy. That thread was solved a long time ago. If you want to reply to someone in that thread, you must quote them and then write a reply. Generally, it looks like you are replying to the original thread creators.