I once earnestly set out to teach a guy for whom I was betaing how to use semicolons. A friend and I sat him down and went through Grammar 101. We explained about periods and commas and run-on sentences. We were clear. We were passionate. At last we introduced (drumroll) The Semicolon. He thanked us profusely and said he really understood it all now.

A few weeks later he handed us a story containing all his old punctuation errors plus a liberal sprinkling of random semicolons on every page. (This was back when fic was printed on actual pages.)

We gave up.