Note in the example above that the first "topic" ("Who is John Mandeville?") has 10 messages and 12 replies.
In the above "screen," clicking on the first topic listed in the previous screen expands the description of the topic and provides access to the "messages" submitted for that topic. To submit a new message on the topic, you would click on the "new" button.
The reply function is triggered by clicking on a button at the bottom of the message you want to reply to:
That is the basic structure--hierarchical--and you will be posting messages and replies.