The “joke” below about the distinction between “can” and “may” crossed my Facebook feed a couple of times this week. The last time I heard this joke I was around 10 years old, meaning more than 50 years ago, so I am a little bit more than surprised that anyone today would comment on the distinction between “can” and “may” in making polite requests, or even think that such a distinction exists. Nonetheless the post has received tens of thousands of likes and shares.
After nearly 40 years of teaching English Language and Literature, I think I can say, based on my own authority and that of most grammar books published in the last three or four decades, that if there ever was a polite-request distinction between “can” and “may” it disappeared at least 40 years ago.
I have a strong suspicion that the reason this kind of false distinction persists is that if you press a less-than-fully competent teacher of English or uninformed speaker of the language to explain the difference between say “going to” and “will” or between “have seen” and “saw” or between “a few” and “a number of” they will retreat into pure nonsense claims that one is more “polite and formal” than the other. Euphemisms and dysphemisms notwithstanding (see the Sour Glossary for definitions), the distinction between polite and less polite grammar disappeared from English usage in the 16th century with the abandonment of the distinction between “you” and “thee” (the equivalents of the French “vous” and “tu”). “Thee,” “thou” and “thine” persisted in prayers, bible translations and poetry, specifically because these words had disappeared from common usage and being rarefied seemed special and more poetic. Ironically, "thee," "thou," and "thine" are the less polite, less formal and deferential forms but the most typical way that Christians address God.
Register is an important concept in linguistics. The concept reminds us that different words and expressions fit better in particular contexts. The language you use with your grandparents will likely be different from the language you use with your friends. If the context is legal or scientific or formal we expect to hear words and expressions that fit that particular context, but the idea that certain common grammatical expressions, particularly modal auxiliaries like “can,” “should,” “may,” might,” “will” and so on, can be distinguished from one another on the basis of politeness is simply wrong.
This being said, the truth about “correct” English grammar is that it is simply a collection of the most recently accepted errors. Just about anything that is now considered “good English” was a mistake at some point in history. You know all those words in English that end in “e” (like “bite,” “kite,” “courage,” “wide”): we were supposed to pronounce those final “e”s, but since most people in the 16th century were mistakenly leaving them silent, the silent final “e” became the correct thing to do. It’s an idea worth keeping in mind if you have ever had your self-esteem battered by a know-it-all grammar mavin, and based on the vitriolic responses to the above post, it seems like a lot of people have.