A forbidden error is a server issue with your permissions. You may want to ask your host for help with it if you don't know how to fix it.
It also looks like you don't have the .htaccess file uploaded to the directory. Additionally, as I already mentioned, you probably need the RewriteBase directive added in:
Code:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /forum/