Probably a stupid question, what is wrong with "Unser Lehrer isst dem Brot"
The case ;-) You need an accusative object here.
Isn't the bread Accusative in this sentence?
If so, it is still das Brot because Brot is neutral, and neutral doesn't change when it's Accusative.
What is the difference between Unser, Unsere and Unserer?
I think its the gender...
It's all about the 4 main german cases.