Well, the mystery is actually why schools stopped teaching grammar in any way that would actually stick, and why it's the smallest words that seem to give the highest number of brain cramps to otherwise literate people.
Ah, welladay, I can do nothing about it except (as is my wont) whinge and snigger.
Oh, and pass on this wonderful video. (I found it over at Huffpost, but I include hit here because I can. It's British, but the lesson's universal. Really it is.)