When you want to respond to a message board post you need to click on the arrow that is with every board post. See the screenshot attached. This will attach your reply to the post in question. This way the person to whom you are writing will be notified that his/her post has a reply.
If you use begin a new thread the post is not attached and goes unrelated to the top of the queue. The person that you wish to communicate may have no idea of the response.
Interrested readers can follow the thread here:
http://boards.ancestry.com/localities.britisles.historical/1...