With my Father's dementia, he has stopped talking for the most part, except for short responses (a word or several words, rarely a sentence). But over the past few months I've noticed that he often repeats what those around him say - and it seems to be comforting to him to repeat what he hears. I've learned that his ability to read (and most likely his comprehension) is far greater than what we've assumed - and that his problem is that when he wants to say something, he can't find the words or phrases or put them together in a sentence in order to communicate. He has gotten so frustrated that he rarely tries anymore.
So about a month ago I purchased a small white board and brought it to his room, and everyday we change the words or phrases on the board and go over them with him. At first I thought he might refuse to do it - sometimes he balks at things that hint of condescension - but instead he's eager to read the words and reading them seem to make him feel better. It's a comfort for him to see the word "potato" and to be able to say that word. We're starting to do sentences this week, and are going to put well-known quotes on the board next. Although he couldn't come up with these words himself, the words are somehow still with him - and we can help him see them again.
I always tell folks working with my dad to not underestimate him. It's what I've noticed at his memory care facility, this awful underestimation of people. Overestimate what they can do and be comfortable with failure - after almost two years of spending quite a bit of time in my father's facility, the one thing I've learned is that everyone there is more capable that I first realized.