Hmmmm, interesting question. How can one determine what a member of a different species "understands?" I assume you are a dog owner like myself. I know when I say to my dog, "do you want to go on a walk" that she gets excited and heads for the door. I infer that she understands what I've said because of her response, which comes from repeated experience with that sentence and tone of mine. Variables would be the words used, the tone of voice, the pace of speech, the movements associated with the words, the time of day or night, whether or not she has been fed prior to the statement, etc. It is always going to be an inference as to what the dog really "understands."