A person with dementia gets confused, forget where he is or who the people are around him. He may think it is a different time, 25 or 30 years earlier. As the disease progresses, the confusion become more frequent and there is more of that than there is normalcy. The Mayo Clinic site has excellent info on this topic.
While this will depend on the individual, an inability to take care of one's needs is most prominent. Other than forgetting persons close to them, they may become confused about bathing, dressing, feeding themselves, ect.
the person can't live independently, vocabulary shrinks , long-term memory will be affected , wandering and sun-downing are present. total dependence on care taker. aggressive , bed-ridden and can't walk.