Dogs communicate in 4 ways, via body language, scent, touch and vocal tonation. In fact dogs read body language better than we do. Your dog more often than not, communicates with you on a daily basis. I bet you talk to your dog all the time, I know I do but how many of you, know what your dog is actually saying to you. Quite often what you say to your dog verbally or what you think they understand, isn’t what your body language is telling them, except for a few words you may have trained your dog to understand, they don’t know what you are saying to them.
The most common thing I hear from dog owners is “My dog knows when its done the wrong thing” In fact your dog doesn’t know, its reading your body language, feeding off your emotions and listening to your voice. If you are angry, your body posture looks more threatening/domineering and your voice may sound more harsh or high pitched.
Here is a very good article on dog body language and titled What is My Dog Trying to Tell Me well worth the read if you want to understand what your dog is trying to tell you.