RewriteEngine On
    RewriteCond %{HTTP_HOST} klasör.domain.com $ [NC]
    RewriteCond %{REQUEST_URI} !/istediğin klasör/
    RewriteRule ^(.*)$ istediğin klasör/$1 [L]
    RewriteCond %{HTTP_HOST} !(^domain.com $|^www.domain.com $) [NC]
    RewriteRule ^\.*$ /error.htm [L]
ErrorDocument 404 http://www.domain.com/error.htm