Hi Ivan
There is a simpe way to do this using "NAT internal servers".
You can find all the required information in Chapter 6 (Layer 3 - IP Services Configuration Guide) subchapter 7 (NAT configuration) of the configuration guide.
Say you have ethernet 0/0 connected to the Internet with IP address x.y.z.t assigned.
Your LAN computers are on a subnet: 192.168.1.0 / 24
Your WEB server is 192.168.1.2
The simple way to do the port forwarding is:
<router> system-view
[router] interface ethernet 0/0
[router-Etherner0/0] nat outbound
[router-Etherner0/0] nat server protocol tcp global x.y.z.t 80 inside 192.168.1.2 80
The nat server command is explained in the command reference manual and the configuration guide.
Hope it helps.
Regards,
Alex