Language is not static The application of grammar is a constant battle between the rules and the real world. Analogously, the dictionary is a reference for vocabulary, but if we are not careful it can be both a strait-jacket to constrain natural linguistic evolution, and a redundant source of stale meaning. Language is not static. I seem to recall that the Random House "Word of the Year" (there's one of my pet peeves - over-use of quotes) for 2009 was "unfriend," and that this was not a new word but one resurfacing after several hundred years. And didn't Shakespeare not only spell his name inconsistently, but also add considerably to the English vocabulary? In a different wise, the most creative musicians of all time (in my book Bach, Mozart, Beethoven) all had encyclopedic knowledge and their creativity came out of intentionally breaking the rules to create new musical language. So it is with words, and to return to the question, grammar. There is a wonderful book by Steven Pinker (I think it is The Language Instinct) in which he argues for the legitimacy of Ebonics and other linguistic forms as evolved forms of English. He takes phrases such as "you and I" to be compound pronouns and therefore serving as both object and subject (ie without requiring transformation to "you and me"). I do jump on my daughter every time that she says "me and my friend," but primarily to help her with awareness of her speech so that she can tailor it to the situation. "Me and Kate" is legitimately the modern idiom, and so in my daughter's milieu it works well. I write novels (one published, another coming soon), so I do spend a lot of time editing my own work. The greatest challenge I find is actually not grammatical, but in communicating feeling and meaning. This requires not blindly following the stolid rules of comma and colon usage, but finely tuning my instincts to allow myself to break the rules when required. My worst grammatical mistakes are actually when I try to stick rigidly to the grammatical rules as I understand them, losing the feeling of the situation which would be better served by the use of assonance, alliteration, rhythm, or other linguistic tools to create points of emphasis.