My dog understood a lot of words. He also understood what I meant when I made a sentence. These sentences were designed for his understanding. Things like "get Stephen" or "squirrel window".
He and I communicated exceedingly well. When he came to tell me to come with him, I understood something very bad had happened. My husband had had a stroke and was paralyzed on the right side of his body. Prompt medical attention reversed the paralysis.
When he came to get me to deliver him dog treats on a silver platter, that wasn't as intense of a communication that was emergency related. He was more elated because, of course, he got the treats. The silver platter was already in use to feed ground-feeding birds in the winter (deep snow).
He understood Dunkin Donuts, and would be at the door waiting for us. When we got there, he would stick his head out the window and give a sharp bark to worker. The worker would say, "Hi Aidan, want a donut hole?" And so it goes ... same at the place that gave pup cups. He was a favorite at those places ... the workers all knew him, plus he was super cute.
He understood words so well, that we started spelling some of them. He caught on to that, and we used synonyms. A cat became a feline, a dog - canine, bird - winged one, etc.
He also had tons of attention from our family of six. Having so many people around meant he was never alone except for once in 14 years.
