Linguists—not polyglots who speak many languages, but scholars who study language—are concerned with describing how a language works. That’s descriptive grammar. Those who teach English are concerned with prescriptive grammar, that is, with teaching the grammar and usage rules of Standard English. But who made the rules? We can easily forget that grammar does not consist of immutable laws handed down by some grammar deity. A living language spoken by sentient beings evolves and changes over time. Most of us know that a double negative is an error in Standard English, but in Chaucer’s time, double and even triple negatives were standard. A widespread “error“ or deviation from Standard English is the phrase “between you and I.” This usage violates the rule that pronouns must be in the objective case when they are the objects of prepositions, so the correct phrase is “between you and me.” Many grammarians assume that the error arises from hypercorrection. As children, every time we said something like “Jimmy and me want to go out and play,” we were corrected: “No, dear, Jimmy and I want to go out and play.” As a result, any time some speakers combine any word with the first person personal pronoun, they make the pronoun I even when it should be me. But Shakespeare used “between you and I” in The Merchant of Venice, and I have read that it was a common usage in Elizabethan English. It is still considered wrong today by most grammarians, but not by all. The cognitive scientist Steven Pinker has written a spirited defense of the usage. (http://www.newrepublic.com/article/77732/grammar-puss-steven-pinker-language-william-safire)
Some say that correct usage is determined by what the majority of educated speakers say. But how do we determine what the majority say? Sometimes it’s easy. Few educated English speakers ever say “She don’t” instead of “she doesn’t (linguists would point out that in some dialects of English, “she don’t” is standard.) The American Heritage Dictionary of the English Language uses a panel of experts to rule on controversial usages, and the rulings are never unanimous. Take the use of impact as a verb, for example. Ninety-five percent of the panel “disapproves of the use of impact as a transitive verb in the sentence Companies have used disposable techniques that have a potential for impacting our health.” But the usage note points out that impact has been used as a verb since 1601 and in its modern sense of “to affect” since 1935. (I still object to the use of impact as a verb for the reason the usage note suggests: it usually appears in a context of horrible jargon and inflated gobbledygook.) Some object to making verbs out of nouns altogether, but it is a natural process in English. The word contact started out as a noun, but who now objects to “I’ll contact you tomorrow”? In 1969, only 34 percent of the panel approved the use of contact as a verb; today 65 percent accept it.
Over time the meanings of words can change. Traditionally, the verb comprise is not interchangeable with the verb compose. The whole comprises the parts: Canada comprises ten provinces. The parts compose the whole: Ten provinces compose (or makeup) Canada. Today many good and respectable writers used comprise in place of compose, especially in the passive: Canada is comprised of ten provinces.
Nonetheless, those who prescribe correct grammar and usage can take comfort in knowing that their advice is infinitely more reliable than advice on nutrition. No grammatical usage has ever changed as rapidly as the respective health benefits of butter and margarine.