I've a Golden retriever who's 12 years old (i.e. towards the end of his life time, although he looks pretty healthy). This's my first dog, following his increasing age, I've learn that his intelligence grew with time... to a surprising level.
I mean, people (and some scientists) say dogs don't understand words but the tone at which one speaks... I find it to be wrong. No matter what the expression the dog can understand -
1) I'm gonna take um for a walk.
2) Who's gonna take um for a walk (e.g. me, my friend or my dad). Well get this -- If someone is talking over the phone discusses that person x is gonna take um for a walk, he goes to that person expecting him a walk.
3) Apart form realizing when we're gonna administer him medicines, he'll grasp us talking and realize it's time for medicines, then soon he'll be under the bed or the table.
4) What sort of food he's gonna get (from curd to milk to soup to bones to choostics -- yup you got that right).
A lot more I can't remember, I also thinks he understands grammar, not only a few words. A few things extra -
Over time he has learnt the packet in which his favorite choostics lie so he knows what I'm opening. He also recognizes it's sound... and various sounds including the vehicles.
He knows who's watching um... even with side eyes... i.e. he recognizes not only the eye, but the pupil of the eye. Actually he can even understand expressions.
All in all, I think what we understand about him (except his health), he understands the same about us.