From your question and comments I think you're a little unclear on what's happening. I'm not sure how to teach you how to run your router, i think you need to talk to someone near you to show you these things.
Port forwarding is when your router allows outside machines to initiate connections to a machine on your network. It doesn't allow any special access to the router internals, just a connection to your inside machine.
The security issue here is not your router, but your machine. Before, it was invisible to the Internet, and therefore somewhat safe. Now the internal machine can be reached. Is your internal machine safe? Is it patched? What's on that port?
As far as your specific questions:
They always know your external IP address. Any connection you make to someone will show your external IP address. I can try to ping the entire Internet (and with a botnet, some people try) and find your external IP address. If you're connected to the Internet, the external IP is exposed. Also, this has nothing to do whether your ports are forwarded or not.
A hacker can not access you through the forwarded ports. But your router may be set up to allow configuration on a web port. How to set this up is different for each router, but make sure anything similar to "allow configuration on WAN" is disabled. Allow LAN configuration only.
This is a hard question. Most routers are just routers. They do not know what an Attacker is. They're too dumb to know "good guy" from "bad guy". Some routers also have Firewalls. They know certain bad guys, and will filter those out. But your router probably has no filters on this port. If anyone tries to connect, they're probably let in.
Also, in your comments, you ask a lot of questions. The tradition here is to take new questions out of comments and as separate questions. Please try to ask your MAC filtering questions in a new question.
Good luck. Security is very hard. Even the experts don't get it right every time.