As Placement Templates
 

Some pretty basic questions here:

Which ad placement template do I use to place ads on the main page of a specific forum -- NOT the forum home page?

Which one to place an ad on the forum home page?

And which one after the first post on a forum?

After the first post is: ad_showthread_firstpost

As for the others it doesn't quite work like that... You probably want to use: ad_navbar_below

But that appears on every page, forum, thread, calendar, etc... anywhere the navbar shows up.

To put different ads in different places you will need to add all the possible ads in this template and use various conditional statements to display the right ones in the right place... conditions such as THIS_SCRIPT == "forumhome" for the homepage and other conditionals for specific forums.

In the VB3 Articles section of this site is an article listing all the conditionals you'll need and how to use them.