Communication
Except human language, dog communication is much more complex, because all the senses are used and involved: smell, sight, sound, touch. Wild canids and most of domestic dogs get intensions and human moods with an instant, since their senses are so refined, that subtle details we miss are clear messages to them.
Body language model is far more advanced than ours. They express their intention or condition by positioning or moving the body or certain body parts: eyes, ears, head, tail...
Complexity of the language is completed with large scale of sounds they manage to produce. Besides all of that, they leave certain messages with scent marks.
It is crucial to understand that placement of some body parts or sound that dog makes has to be perceived in certain context, grasped as a signal in total, in order to figure out properly what it is that dog wants or feels, which is necessary for complete relationship between human and the dog, but also for understanding the dog's needs. Also, it will prevent unpleasant situations and bites due to misinterpretation of dog signals.
Unlike wild canids that can demonstrate handful of signals – a wolf can make 20 different signals with its head alone – domestic dogs because of the human interference, creating breeds, cropping of ears, tail docking, cannot send as much signals or signals they send are not correspondent to dog's intentions.