why can't ftp through firewall?

Now i meet this question:
The host in my network with private IP as 192.168.1.41/32 wanted to give FTP services to Internet users.It is behind the firewall.So i need to configure it with port redirection.The config table like as:
202.38.*.*/32 21 mapped to 192.168.1.41/32 21
Then I ftp the 202.38.*.* from Internet,i connect it successfully.
But when I use the command 'ls',it cannt show the files in FTP host.
It printd:"turn to passing model",and i cant see nothing.
The ftpd server is Server-U in windows2000.
I know the FTP command port is 21,but transfer port is various per times(is it?)
I want to know what the reasons is.Thanks more.

It may be a setting in your FTP client. Try connecting from your command line. (c:/ftp host) and once you are logged in, see if you can do the ls command and see your files. If that works, check to see if your FTP client is set to run in passove mode. If so, change it so it is not.

On the server side, transfer port is 20.