Re: How do I teach my dog to take treats nicely?
Instead of taking the treat away (which is exactly what the dog fears, or at least expects, and why they tend to be so grabby), I show the dog that he has to EARN the treat by being gentle.
Take a high value treat that you can cut into a strip. A piece of cheese, deli meat, or dehydrated meat snack. (I like Merrick lamb lung for this). Put most of it in your palm, close your palm, so that a teeny tiny bit of it is sticking out by your thumb and index finger. Show your hand to your dog. If he gently nuzzles your hand or does nothing, immediately give him the treat. If he mouths your hand, hold tight on to the treat, and tell him Gentle.
Let him work at it. At first, he'll probably be pretty assertive (and your hand will get banged up). Keep telling him Gentle. But he's aiming for such a tiny piece, that eventually, he'll just use his tongue or lips. When you get ONLY lips and tongue (no teeth or jaw), immediately hand him the snack. Repeat about 5 times. Then stop for that session.
Don't attach this to any other training. Just do it a couple times a day. Whenever you want to give him a treat otherwise, put it on a plate (or if you don't mind your dog eating off the floor, you can put it on the floor), make him wait, then release him to eat it. In other words, he doesn't eat unless he shows patience every time he gets a snack.
Yes, you're going to have to slow down on training sits/downs/etc for a day or so.
Within a few days, your dog will be taking treats gently out of your hand, because he knows that the ONLY way to get your hand to open is for him to gently take them. There is NO other way. (You can then start to refine his approach further. Smaller treats and treats from a more open hand or just your fingertips, whatever.)
You note I don't ever use the word "no." The pup just doesn't get what he wants. Our puppies are smart. They do what works for them. Every time. As long as we're consistent too.