I need a little advice......
My 10 mo. GSD has been doing very well with basic obedience with a couple exceptions.
Let me set this up a little.....
When he was about 6 months old I started using food to introduce commands and he seemed to learn rather quickly. When I thought he knew the command, I gradually moved away from food and used corrections when he didn't listen. That just didn't work for him. He seemed to get discouraged and confused when corrected and praise didn't do the trick as a motivator. So I took a couple steps backward and started using food again. Again he was doing well with food as a motivator. During this time I was also using a ball on a string to build his drive and focus. When I thought his drive was high enough, I started using a ball instead of food for a motivator. This too seemed to work rather well. Currently I use food during formal training and I give him commands to make him work for the ball when we are playing. I don't have a problem with using treats or a ball as a motivator because he really seems to enjoy the training.
Here's the question.....I want him to know that when I tell him to do something or give him a command that he "must" obey. Right now it's just a game to him, which is great. What is the best way to get the results without discouraging him.
Another question.....We also have a cat and he is obsessed with her. He knows that he is not allowed to chase her, but if I leave the room or turn my back for a second, he goes after her. He has never hurt her, but I am afraid that he may in the heat of the chase. He knows that he is not supposed to do it because when I catch him in the act, he will run the other way and lay down in his bed. I have tried correcting him for this behavior, but nothing seems to help. I've considered getting an e-collar to zap him when he approaches the cat. Any advice?

Continue to use motivation in your training, ask the dog to give you more and more before the reward is given. Simply with hold the reward if the dog doesn't give you the correct behavior. Only add corrections when the dog is very solid, on any certain behavior. I would prefer that a correction is never given, but some times distractions will cause disobedience.

About the cat thing. According to you the dog already understands that he can't chase the cat around you. So I don't know what you will achive with an E-collar because the only time you can give the correction is if you are around the two.
Go find a big bad alley cat, let him get his butt tore up that will give him doubts about chasing cats.

The Alley Cat thing was tounge and check, I hope that no one would actually do that. Even though it might work.