It’s true. You may be confusing your dog by talking too much during training sessions or while practicing training at home.
When your dog is learning a new behavior such as “sit” they actually learn the word “sit” last. Saying “sit” to your dog out loud is what we trainers call a verbal cue. The verbal cue is the last thing your dog will learn in training. That just how doggy brains work. Your dog will first learn a visual cue, usually a lift of the hand. It’s only after your dog understands the visual cue (your hand gesture) that your trainer will add a verbal cue (the word “sit”).
It generally goes something like this:
Step one: Show your dog that you have a treat in your hand. Your dog will want the treat so will focus his attention on it.
Step Two: While holding the treat, raise your hand up in front of/above your pup’s head. The idea is to lure your dog’s attention to follow the treat with her eyes. As you raise the treat, your dog will most likely lower his bum to the floor while raising his head. Once your dog is in the “sit” 
position, give him the treat and praise him.
Step Three: Practice this! Your dog will associate you raising your hand up with her sitting and getting a treat. You can stop holding the treat in the hand you raise when you think your dog is ready (i.e. she is understanding your visual cue to sit), giving the treat with your other hand as a reward once your dog sits.
Step Four: Now you can add the verbal cue to sit by saying “sit” out loud when you raise your hand in the visual cue (yes, keep using the visual cue, you are now just adding the verbal cue). With practice, your dog will associate the spoken word “sit” with the action of sitting and, voila, you’ve 
taught your dog to sit!
Here’s where people often go wrong:
For us humans, the word we use to ask our dog to do a particular behavior is what seems most important because we are language focused. The thing is, that isn’t how dogs learn. If you were to follow my instructions above except that instead of using the vocal cue “sit” you use the word “peanut” your dog will learn to sit when you say “peanut” out loud.
To you, sit means sit. To your dog, sit (or peanut) only means what you teach her it means. That’s why it’s important to teach your dog based on a visual cue first because that’s how your dog learns best.
Another point to keep in mind is that you don’t want to talk to much. What I mean by that is, when you are teaching your dog a new behavior and you come to the point where you want to add the verbal cue, don’t overdo it!
To your dog, “sit, sit, sit, sit” is a very different “word” than “sit.” If you say “sit” and your dog just looks at you quizzically, just wait (silently) while your dog works out what you want in his own mind. If your dog doesn’t get it yet, retrace your training steps by adding back in the visual cue and keep practicing.
If your dog doesn’t do as you ask the first time, she likely doesn’t understand what you want. Repeating the verbal cue multiple times will just confuse your dog more and end up frustrating both of you.
On the off chance your dog just didn’t hear you, I tell my clients to count to ten in their minds before saying the verbal cue again. That way your dog has time to think, but if he didn’t hear you, you’ve said it again without waiting too long.
Your dog loves to engage with you and have your attention. Keep training fun and feeling like a game by being patient, training in a way that works for your dog’s brain and keeping your voice and body language upbeat.
You may be surprised by what your dog is capable of, how much fun you can have training together and even what a great dog trainer you turn out to be.