The rule I sent was a modified version of one I'm using. What it's supposed to do is capture the entire request (for example - page.phtml) and then append /forums.php to it - so it would become page.phtml/forums.php.
You may also want to write it this way:
^(.*)\.phtml$ $1/forums.php - which should strip off the .phtml
RewriteRules can be tricky. Be patient, you'll get them working.