RewriteEngine on
# This isn't always needed Remove the # if it still doesn't work abnd try that
# RewriteBase /
RewriteCond %{HTTP_HOST} ^mydomain\.com$ [OR]
RewriteCond %{HTTP_HOST} ^www\.mydomain\.com$
RewriteRule ^(.*)$ "https\:\/\/mydomain\.com\/forums\/$1" [R=301,L]
# if needed, add for SSL if non-https is requested
# RewriteCond %{HTTPS} off
# RewriteRule ^(.*)$ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]