Anyone can train/teach a dog a number of behaviors and tricks. The main ingredients to your dog learning a behavior are the right motivation (reward) and consistency. From a puppy to an older dog, no dog is untrainable/unteachable. Here's some advice and tricks to teach your dog a new command. Please note that my terminology of the use of "dog" does not mean that you cannot use these techniques with puppies.

Dogs have a natural tendency to push in the opposite direction against pressure that’s being applied to certain parts of their body. For example, when they feel the tug of the leash on their neck, they tend to pull forward. When you push on their back and ask them to sit, they resist. That's why having the perfect setup for walks can make such a difference.