Dogs pant slightly when they are happy or energetic. In fact, it may be so unnoticeable that you might classify it as light breathing. Dogs communicate with their physical presence however, there are some sound signals you can rely on to interpret what they're thinking or feeling. Until then, expect subtle verbal cues for decoding canine body language. If your dog one day starts speaking to you, pull out your phone and take a video, and politely take to the celebrity status.
While your dog may seem to understand your words, especially if you've put in the effort to train them on simple commands such as sit, heel, and stay, you also need to work to understand and translate canine body language. Words may be important for humans to communicate, but dogs convey emotions through body language and sounds.