I know because, in a July 2017 interview with New York Times  reporters, he said:
So pre-existing conditions are a tough deal. Because you are basically saying from the moment the insurance, you’re 21 years old, you start working and you’re paying $12 a year for insurance, and by the time you’re 70, you get a nice plan. Here’s something where you walk up and say, “I want my insurance.” It’s a very tough deal, but it is something that we’re doing a good job of.Pity the person who had to transcribe that outpouring of nonsense. I could quibble with the punctuation, but let’s be honest: you could move commas and add dashes all day and never uncover what Trump was trying to say about preexisting conditions. What is clear is that the man who holds the presidency and leads the Republican Party, the man who never stops shouting and tweeting about how Obamacare is a disaster, thinks health insurance works like a pension. He thinks it costs $12 a year. And he is this ignorant after months in office. He is so completely ignorant he doesn’t even know it’s a bad idea for him to talk about it.
Perhaps more attention should be paid to this alarming fact? Perhaps it merits a follow-up question or two when Trump, without any prodding, reveals his fathomless ignorance in areas of great national importance?
There is no precedent for covering a president as incompetent as Trump. And so reporters go on treating him like they would any other commander-in-chief, instead of making his unprecedented incompetence the headline story it should be. In this case, no one replied, “Excuse me, Mr. President, but what you are describing is not how health insurance works”—at least not on the record. Instead, reporter Maggie Haberman said, “Am I wrong in thinking—I’ve talked to you a bunch of times about this over the last couple years, but you are generally of the view that people should have health care, right?”
It used to be safe for reporters and pundits to assume that the president was reasonably well informed. Now that assumption is no longer warranted. Professionalism cannot require ignoring what is obviously true. Trump’s unfamiliarity with the basics of health insurance ought to feature prominently in any future reporting on his administration’s efforts to influence health-care policy. Other Republicans should be asked to address it. And on other subjects, his competency should not be taken for granted. There is no question too simple to be worth asking.
Even before he took office, news broke that Trump was refusing the intelligence briefings meant to get him up to speed. When challenged, he explained he didn’t need them: “I’m, like, a really smart person.” He may really believe that. The rest of us have ample reason not to. Journalists who proceed as though Trump knows his stuff are engaged in play-acting that only obscures the truth.