RewriteEngine On

# RewriteBase /
RewriteCond %{REQUEST_FILENAME} -d [OR]
RewriteCond %{REQUEST_FILENAME} -f
RewriteRule ^ - [L]

RewriteRule ^(.+)$ shortn/index.php?code=$1 [QSA,L]
#It might be this one, note sure if you need a / before shortn
# RewriteRule ^(.+)$ /shortn/index.php?code=$1 [QSA,L]