You can send a private message to a forum member by clicking on the member’s username then the “New private message”. The message will appear in the member’s forum inbox (accessible via the envelope icon- the red number shows unread messages). They will also get an email notfication (default) unless they have opted out of message notifications.
To keep a reply to a topic private, you can tick the “Only share reply with” box. Then your reply will only be visible to that person (and only when they are logged in). Please use only when necessary as the purpose of the forum is to add to our collective knowledge.
NB If your message is confidential, please use conventional email, using the private message facility on this site to obtain the relevant email address.