To commemorate H.L. Mencken's birthday on this site,* it seems particularly fitting to repeat a salient passage from The American Language, a caution to all those who hold forth and English grammar and usage:
The error of ... viewers with alarm is in assuming that there is enough magic in pedagogy to teach 'correct' English to the plain people. There is, in fact, too little; even the fearsome abracadabra of Teachers College, Columbia, will never suffice for the purpose. The plain people will always make their own language, and the best that grammarians can do is to follow after it, haltingly, and often without much insight. Their lives would be more comfortable if they ceased to repine over it, and instead gave it some hard study. It is very amusing, and not a little instructive.
If you are inclined to dismiss Mencken, this blog has previously risen to his defense.