Dementia isn't actually a single disease, and it doesn't affect everyone equally. His walking and motor function doesn't seem impaired, but for some dementia patients, problems with walking are the first signs because Parkinson's can accompany dementia.
For whatever anyone believes his words and ideas had, Biden used to be a world class talker and schmoozer. He had a creepy vibe about him, but in the pre #metoo era, it was mistaken as charm. He was natural at asserting his position, going off topic, and being fluid in deflecting counterarguments. Even if his arguments were garbage, he was absolutely convinced that he was still correct and lots of people would mistake that confidence as their own misunderstanding of what they believed. Even in a portion of his prior capacity, he is still quite capable of forming sentences most of the time.
The diminishment is seen in how he handles new topics for discussions, challenges to his positions or previous statements, and actual demonstrations of him thinking about things rather than reciting talking points.
Watching his current speeches and news conferences it's mostly obvious when you can see him try to use an old rhetorical tricks and reach mentally for something that isn't there anymore. He comes across as more frustrated, he can't meander then come back to a topic like he used to so effectively. He knows that if he meanders, he's going to lose it, and he's not comfortable sticking to a script.
about the sentences...if he is reading from a prompter or listening to an ear piece all the time he may seem to be forming his own ideas etc...
he could use an ear piece for the debates...
he read the prompter slowly today...he could continue to speak slowly and people might think he usually speak that way or that he was carefully answering the question..
also by speaking slower he is less likely to stumble over words and sound ill...
Maybe we’ll get lucky and Girlfriend will be the one to feed him the answers...