Let’s take a look at how prescriptivism can interfere with establishing whether or not an expression is standard English or not. Let’s say the question is whether a word is an adjective or an adverb. You may want to use a word that functions as one or the other but cannot decide whether what you see written is being used as an adjective or an adverb.
One way would be to use a definition of the part of speech to see if the use you are looking at fits that. You might see, “He hit it big” and wonder if big is being used as an adjective or an adverb. You mutter to yourself, “He hit it well” and you know well is an adverb. But then you can say, “He hit it good”. You look up good and find it can function as an adverb. You also have the feeling that, “He hit it good” is not standard written English and that to hit something big is not really using big as an adverb.
To further confuse the issue, we have the rule that adjectives end in -ly, not true but commonly taught. We run into problems with that rule because some adverbs in Old English ended in an adverb-making ending -e which dropped off in the course of the development of English. An excellent example is “slow” which “grew” an ahistorical ending -ly, so now we have both slow and slowly used as adverbs. This misunderstanding so screwed things up that it is difficult now, at least in some contexts, not to use slowly, ahistorical as it may be. “Please drive slow” and “Please drive slowly” are both heard but some crusty English teachers will declare the former wrong.
But because most people do not study grammar, prescriptive or otherwise, they continue using forms from older stages in the language without realizing they violate some rule of prescriptive grammar. Most people pay attention to prescriptive grammar rules because that is how we establish a standard, we lay down rules. What most people, including English teachers, have trouble understanding is that formal written English is where you use the standard, not in everyday speech aka colloquial speech.
So people understandably begin mistrusting their grasp of their own language b/c the formal written standard deviates enough from the spoken standard that some artificial rules must be learned so they can be applied in the appropriate circumstances. Unfortunately, teachers who do not understand how to develop language ability in their students just leave students confused or, in a few cases, following strange rules like using the subject pronoun in a predicate nominative when it just doesn’t work, even in formal written English, e.g. ” The woman in the picture is I” or “The woman in the picture is she” when the object pronouns me and her work just as well in the formal written as in the colloquial.
The question is, how do we teach the standard to students who do not hear it at home? Teaching a handful of absurd rules like the classic split infinitive instead of helping students see how a so-called dangling participle can confuse the reader or not using commas to indicate a non-restrictive adjectival clause, e.g. “The enemy sank the ship which was new” vs “The enemy sank the ship, which was new,” results in very different meanings.
The best way to teach formal written English is to have students read and then slowly (slow?) begin writing. But prescriptive teaching can be helpful to all students, no matter how standard their colloquial language is. If we continue teaching prescriptive rules unthinkingly, we hog-tie our students b/c the rules make no sense and they decide they just don’t know English, “proper” English. I recall ferocious battles on a listserv over the use, in English, of “good” in response to “How are you doing?” The answer should be an adverb answering how and good is not an adverb. Well, it is used in standard English as an adverb – so there!
And then “How do you feel?” elicits an adjective in response: “I feel good”. Most people have an intuitive sense of this, but many dialects of American English use non-standard forms like double negatives. While double negatives are standard in many languages and was in English – thus its continued use in the colloquial non-standard language – the prescriptive rule has been so well promulgated that we just have to accept that double negatives are – unacceptable.
These thoughts are occasioned by the frequent confusion I hear from people who are quite well educated and do speak the standard, but have no knowledge of English as an object of study and do not even know the prescriptive rules. Someone just said today, “I lay down and when I was laying there……..” Such a verb in transition, where the intransitive and transitive forms are not only merging but lie has irregularities in its formation, needs to be taught prescriptively.