Just to add my opinion on "no":
No, to humans, is a negative word. It can be a negative answer to a question, or it can be restrictive, it can be a reprimand, etc. But, it's ALWAYS negative. It's NEVER positive.
And, as many have said, "no" has so many meanings and can be used in so many situations.
When we tell someone, or even a dog, "no" what we really mean is "that's wrong, don't do that" or "that's bad" or any number of things. The thing is, dogs don't think in terms of right/wrong, or good/bad. They DO think in terms of what works for them and what doesn't work for them.
So, there's this disconnect between us and dogs, when we say "no" we are usually not just trying to interrupt, we are also subconsciously attaching a judgment (negative). And, humans can't seem to understand WHY their dogs don't stop when they tell them "no" or why they don't understand when they tell them "no". The reason is, they're not getting the hidden, attached meaning.
To them, "no" means nothing more than "eh" or "ah ah" or "uh uh" or "hey". But, many of us, when we say no, we expect them to get that no means it's wrong. All they get is that you made a sound.
Now, if you make the sound, and then go on to give further instruction, you have a chance of giving them a better concept.
Plus, lots of times, if a dog doesn't respond right away, "no" becomes "no no NO NO NO NONONONO!" which makes even less sense to them!
