Code:
Options +FollowSymLinks 
RewriteEngine On

DirectoryIndex mvm.php
ErrorDocument 404 /mvm.php
RewriteRule ^chessdatabase/$ /mvm.php [L]
RewriteRule ^searchposition/(.*)/$ /search-database-position.php?fen=$1 [L]
RewriteRule ^([0-9]+)/searchposition/(.*)/$ /search-database-position.php?fen=$2&page=$1 [L]
RewriteRule ^chessgame/(.*)/(.*)/([0-9]+)/$ /chessgame.php?gameid=$3 [L]
RewriteRule ^onlinechessdatabase/$ /search-form.php [L]
RewriteRule ^onlinechessdatabase/([0-9]+)/$ /search-form.php?page=$1 [L]

# If requested resource does not exist as a file
RewriteCond %{REQUEST_FILENAME} !-f
# and does not end with a period followed by a filetype
RewriteCond %{REQUEST_URI} !\..+$
# and does not end with a slash
RewriteCond %{REQUEST_URI} !^(.*)/$
# then add a trailing slash and redirect
RewriteRule ^(.*)$ http://www.gensunasumus.com/$1/ [L,R=301]