Just the one page? Or do you want rewrite a whole series of pages? Instead of using a filename without an extension like /index, I would recommend adding an extension, like .html to it so its /index.html
The one page is pretty simple. Just create a .htaccess file in your root directory with the following:
Keep in mind though if you're using ModRewrite to direct to a file in a different directory, your HTML may have to be modified if it uses relative paths for the images. Else it will be looking for your images in /images/ instead of /forums/images/ . Same for links. Maybe if you let me know what your intended purpose is I could better assist with some rewrite advice.