mod_rewite rule

When a user requests the url:

https://www.example.com/flag/my-post

I want the server to deliver data from this adress:

https://www.example.com/my-special-page/?show=my-post

in my .htaccess i found the following code:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /ny/
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /ny/index.php [L]
</IfModule>
# END WordPress