There are foods that dogs can’t manage. That’s not to say they won’t try, but some kinds of food which might be completely benign to people can kill dogs.
This is, by no means, a comprehensive list. Check with your vet if you’re in any doubt.
Things dogs should never eat:
Bacon and fatty meats,
Salt and salty stuff,
Milk, cheese, ice cream, dairy, generally (lactose is complicated),
Grapes and raisins,
Seeds and pits from fruit,
I know some of this because of a very helpful poster at the veterinary surgery, and some of it because Doobie used to go bin-diving regularly which led, inevitably, to emergency veterinary appointments, a decent round of stomach-pumping and several terrifying invoices.
However, dogs can cope with decaffeinated tea, as long as it’s weak. You can add a splash of goat’s milk (easier to digest than cow’s milk, but don’t go overboard with it) and cooled to hand-temperature.
Doobie hardly drinks at all. Frankly, it’s weird. There’s fresh water out all the time, dotted all around the house, and yet, he doesn’t drink. Unless I make him a cup of tea.