Obviously, dogs are not born understanding our language. However, by being consistent and keeping things simple they can learn many words. Just as a baby learns words have meanings, so can your dog.
However, dog training is much more than your dog just following commands. It is learning to communicate with your dog. It is showing him what you want and expect from him in a way that your dog can understand. You and your dog will be happier if the dog lives in an environment where he knows the rules. Consistency is a priority. Your dog must believe the rules will remain the same from day to day.