Are you having trouble learning how to teach sit when it comes to a suddenly aggressive dog
? A well behaved dog will not normally have trouble meeting people face to face.
Does the dogs ears push forward his tail raise high over his back? Act out towards others? This is a common for a suddenly aggressive dog to do. Your dogs behavior patterns should include tail wagging at body level, sniffing behind and the capability to meet other people and dogs face to face. You need to observe what your dog does before you can learn how to teach sit him.
You may or may not be able to learn how to teach sit if your dog is an overly suddenly aggressive dog. If he bits and lunges at you for just trying to teach him anything, then you may want to seek professional help. But if your suddenly aggressive dog only acts this way around strangers, then by learning how to teach sit, this may be the solution to your problem. I can teach you a really easy method. The first thing you do is take one of his favorite treats or toys and hold it in plain sight, semi close to his nose.
Take the treat and bring it over his head to his tail area, while you are saying the command. He should sit naturally for a second. You may have to push his butt down slightly. Once he sits, give him the treat or toy and tell him good job. Only treat him for being calm, if he barks, tell him no.
Make sure he realizes that a suddenly aggressive dog does not get a treat. You learn how to teach sit with repetition. You should learn how to teach sit in just 5 minutes a day. That should do the trick for your suddenly aggressive dog. I included a great link to a free dog training lesson at the bottom. It may help you more then I can.