I wouldn’t like to hastily jump on the “Trump is mentally ill!” train, but I did find this article interesting.
https://www.statnews.com/2017/05/23/donald-trump-speaking-style-interviews/
Research has shown that changes in speaking style can result from cognitive decline. STAT therefore asked experts in neurolinguistics and cognitive assessment, as well as psychologists and psychiatrists, to compare Trump’s speech from decades ago to that in 2017; they all agreed there had been a deterioration, and some said it could reflect changes in the health of Trump’s brain.
In interviews Trump gave in the 1980s and 1990s (with Tom Brokaw, David Letterman, Oprah Winfrey, Charlie Rose, and others), he spoke articulately, used sophisticated vocabulary, inserted dependent clauses into his sentences without losing his train of thought, and strung together sentences into a polished paragraph, which — and this is no mean feat — would have scanned just fine in print. This was so even when reporters asked tough questions about, for instance, his divorce, his brush with bankruptcy, and why he doesn’t build housing for working-class Americans.
Trump fluently peppered his answers with words and phrases such as “subsided,” “inclination,” “discredited,” “sparring session,” and “a certain innate intelligence.” He tossed off well-turned sentences such as, “It could have been a contentious route,” and, “These are the only casinos in the United States that are so rated.” He even offered thoughtful, articulate aphorisms: “If you get into what’s missing, you don’t appreciate what you have,” and, “Adversity is a very funny thing.”