Hey,
This may seem like a wired question, and tried asking this question in the phpbb forum, but got new responses so here goes,

In my forum, I have a ad under my first post too like the one below this post, but right now, it shows a gap in between the first post and second post (or 1st reply), so my question is, how where you able to insert it into a table to make it match with the rest of the forum.

thanks in advance?

It's very hard to answer without knowing at least the script that you are using and better with a link to see it directly.