You need to create a ".htaccess" file in your root www directory. It needs only one line:
Redirect /forums http://forums.test.com
You can create this .htaccess file using Windows Notepad, just specify the file name WITH QUOTES in the filename field: ".htaccess" and then you can upload it to your site using your favorite FTP program.
You can also create the file directly on the server if you log in with SSH, but I think Notepad+FTP is easier for you?
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