Dogs don't speak human. They learn by what we do, not by what we say. So if you sometimes mean "sit" when you say "sit", and other times mean "go away" when you say "sit" don't be surprised if the dog never learns what "sit" means to you! Dogs love routine. They love getting up at the same time every day, and you coming home at the same time, and having same word mean the same thing every time they hear it. They like knowing whether when you use a word whether you mean "you must" or whether you are saying "you may". Does the word "sit" mean "You must sit" or does it mean "sit if you want to, otherwise don't bother"? The dog wants you to mean it the same way every time.