A describing how people speak and a telling them how to speak are doing different things with the same vocabulary.
The confusion between them is old, and it survives because both use the word 'correct'.
For the linguist, a form is correct if native speakers produce it systematically.
For the tradition, a form is correct if it to a standard that some authority has recorded.
Neither definition is foolish, and the arguments become unproductive only when the two are used in the same sentence.
Most rules that survive in schoolbooks were invented in a short period and by very few people.
The on splitting an , for instance, was imported from Latin, where the is a single word and cannot be split.
English writers had been splitting s for four centuries before anybody objected.
This does not settle the question, because a rule need not be well founded to be socially real.
A speaker judged by an employer against a standard she considers is still judged.
The linguist who tells her the rule has no basis has described the world accurately and has not helped her.
The useful position is therefore neither pure description nor obedience, but a distinction between two questions.
What does this form actually do in the language, and what will it cost the speaker in this particular room?
Those questions have different answers and are frequently confused by teachers who mean well.
A rule presented as a fact about English invites contempt when the student discovers it is not one.
The same rule presented as a convention of a particular is accepted without difficulty.
Students are not resistant to conventions; they are resistant to conventions dressed as laws of nature.
Teaching the difference costs one lesson and removes an argument that otherwise lasts a lifetime.