Clicking the reply to this post button in a post display will open a new page showing the current message and a form for submitting a reply.
At the top of the form it will display your username. If you are not logged in (browsing anonymously) nothing will display here.
Beneath that is a field for the name to display. If you are logged in, your username will appear here. If you wish your name to be displayed in the forum differently, you may change it here-- but the new name, no matter what it is, will still link back to your user profile.
Beneath that is the subject line. The system will automatically take the previous post's subject and pre-pend it with Re: (if it does not already contain Re:). You may edit the subject line if you wish. If it is longer than 25 characters it will be shortened to 25 in the forum index, but the full subject will be shown in the message display.
Beneath that is the textarea for the message body. By default this will contain the contents of the previous post, wrapped to 65 characters per line, with each line preceded by : . Underneath the textarea is a list of allowed HTML tags.
If you delete all the text from the body, the tag [NT] will be appended to your subject line to let people know there is No Text inside.
There are two buttons at the bottom of the form. Preview will show you what your post will look like when it is posted. Post will add the post to the forum.