There is almost a zero chance Trump will run in 2028. He is old and that would be his third term.

Prior to FDR, there was a convention -- but no legal restriction -- that Presidents would only serve two terms. This stemmed from some statement that Washington made -- he was getting really old, not far from dying and didn't really want to be President in the first place, but got repeatedly argued into it, as he was one figure that had widespread support across the country.

FDR broke that convention, served three-and-some terms and died in office.

After that, Republicans passed the Twenty-second Amendment prohibiting Presidents from serving more than two terms.

Today, you can only do two full terms. If you succeed an existing president -- like, you're Vice President and the President dies in office, you can do part of that, can't recall what portion is permissible. So Trump is out after this term.

pokes around

Yeah, can be any partial amount of their term, isn't like "over 50% of their term" counts as a term.