Greetings!
I cared for my mother for 13 years with Alzheimers and I work at a nursing home with primarily dementia patients.
The main "rule" we follow is to Validate with Kindness. It's nearly impossible to make hard and fast rules since dementia changes day to day. We have to be careful with our tone of voice which is difficult with so much aggravation.
For example. My mother caught me off guard one day when she was in nursing home by asking "When can I go home?" She clearly did not remember that she had sold her house.
I simply said "I'll have to check with your doctor." This satisfied her. I did not have to say "you are never going home" or "don't you remember you sold your house?"