Forum Section A: Can only be viewed by Admins Forum Section B: Can only be viewed by Admins and Moderators Forum Section C: Can only be viewed by Admins/Moderators/Members Forum Section D: Can be viewed by anyone
David, a private forum is no different in how it works than an "open" forum. The only difference is the admin is able to restrict or choose who is able to view and/or use the forum. This is done within the "editing" - (use the pencil icon on the right of the forum's title line) to make any changes to the forum's properties.
All the ways of creating topics, and replying to such, is covered in the FAQs of all ForumCo's forums. You'll find the a link to the FAQs up toward the upper right of all the forum's pages (Up near the "Login" option). You can also find discussions on any particular subject by doing a "Search" of this support forum.
It makes no difference if it is a "Private" or "Open" forum, create your topics by using the "New Topic" option within each and every forum of your over all forum.