Beyond Grammar and Into the Social Codes
If you have ever taught English, you might know that "May I have x" is a polite form. This really is not about grammar. It's about how certain expressions are used in certain situations and also functions as markers of a social code.
This is not about being technically correct, it's about being the most polite (one can be.) There are many variations on how to ask to be served something in a restaurant, but as far as may/can goes, the utterance marked as most polite is:
1) "May I have [dish or food] please?"
Here, "May I have [dish or food]" means: I am ordering this dish or food.
That said, one often hears "Can I have x" when ordering food, and it's acceptable and fine in contemporary American English since everybody uses it but will not pass an advanced politeness test.
Permission: May I x is also used to ask for actual permission to do something:
- May I leave my car in front of your house for the weekend?
You will hear, however, people asking for permission and saying:
- Can I leave my car in front of your house.
And,naturally, this use of "Can I" for permission has zero to do with having the ability to do something as is: "Can he ride a horse or will I have to teach him how?"
In the US, you will hear guys (yes, mostly guys) go into a pizza shop and say: "Can I get (pronouncing it "git") a slice of pizza?" That's fine in a pizza shop. It is typically working class and you probably wouldn't hear "Can I git" in a five-star restaurant. Here's the nasty truth: There is a class issue here. People who say "May I" in a restaurant are immediately recognizable by others who would say that. There are all sorts of educational and social status implications in it. No one like to admit this but it is true.
2) "Shall I [do x for someone]." Shall I open the window?
Shall I [do x for someone] means: Do you want [person or pronoun] to do [something] for [someone].
"Shall I" is used to offer to do something for someone or to suggest doing something for someone.
My answer is to this question is not about grammar, but about politeness as strategy.
Politeness Strategies in English Grammar
by Richard Nordquist
Updated December 29, 2017
In sociolinguistics and conversation analysis (CA), politeness strategies are speech acts that express concern for others and minimize threats to self-esteem ("face") in particular social contexts.
Politeness as strategy, not grammar