Leonberger are a dog breed that can only be found in Germany, but due to the world becoming a smaller place, the breed is being spread to all parts of our planet. These dogs are special for many reasons, they eat a lot, love their owners and are good with people and children. But the most fascinating thing about them is that they are quite easy to train but, to keep them in a nice shape, they need to have a lifelong training going on.
We will skip the main commands every dog should know, like sit, roll, come and similar ones, and go over to the more advanced and useful ones. Many dogs, among this breed also, are often moody, they behave well when they are in a good mood and do it differently when they are not. There are a few things you can do about that, first of all, make sure to get the dog as a puppy and train it from the beginning on. Teach it how to respect you and let it know who the boss is, but don't be afraid, it will still love you. Then socialize the dog, make sure you Leonberger is well behaved when it is around people and other dogs. Give it time to figure out what is right and what wrong and help him to do so, with treats.
One useful training you can do with your dog is to learn it to refuse food, or to stay calm in any situation. The signal for that should be verbal like 'leave' or 'quiet'. Ask one of your friends to help you by taking a treat in his hand and calling the dog in one situation and then calling the Leonberger without having food for him.
Let the dog go to the person and take the food, but then say 'leave' and hold a couple of goodies in your hand and call it. Do it until you are sure it is time to do the training together where both, you and your friend, call the dog, with you saying 'leave' afterwards. If you are doing it right the dog should obey and come to you every time you say 'leave' even if someone has a treat for him.