There's a lot of good advice here; I think your comments will be very useful to language learners. I think you have misunderstood the prescriptive/descriptive distinction, though. I know the distinction you are making, and it's a good one, and I regret that I don't have apposite words to use for it (perhaps deductive and inductive are close). Unfortunately, prescriptive and descriptive, as presently used in linguistics, aren't the right terms; they refer to a different dichotomy from the one you're discussing. Fearing that you'll hate your fanboy for quibbling ... ---ACW