A student in an online copy-editing course I'm teaching — we'll call her Jane — was once a very diligent high school student. She paid close attention in English class and carries with her to this day the lessons she learned there.
In fact, she can still hear the voice of her old English teacher in her mind. And it was that voice that, during a recent discussion on real-world editing, led Jane to ask me the following question:
"How about the rule to never begin a sentence with 'and' or 'but'? I know that for formal writing it's not acceptable, but how can you tell when it is OK to use in informal writing? It's one of those rules that was pounded into my head. So even when I break the rule in an informal e-mail, I hear my high school English teaching saying, 'That's not right, dear.'"