May you please explain this?

At a family dinner, my 18-year-old niece asked my sister, "May you please pass the salt?" My sister said that she was impressed with her daughter's politeness, but that that particular wording was not correct. My niece said that she had been taught to say that (by whom she could not say, but probably by her father—the parents divorced years ago.)

Despite our best attempts, we were unable to convince her of the illogical nature of the "May you please" construction.

How would you explain it to her?

Solution 1:

I think that there is possibly confusion here between may, can and would.

It is possible that she once used to say expressions like:

Can I have ...
Can I get ...

and was taught that it was more polite to use may rather than can in that context.

Although strictly, can relates to the ability to do something, whereas may concerns permission to do something, can is often used instead of may in constructs such as the above. That works acceptably in the first person: "Can/May I [do something]".

Can and may are also used interchangeably is expressions such as:

You may leave now.
You can leave now.

Although both are intended to give permission, again - strictly - the latter relates to the ability to leave (as if the person were previously locked in!).

So can and may are often interchanged when asking or giving permission, but, in your niece's case, she was doing neither: she was requesting someone else to do something. In that case, can still works, strictly meaning "Are you able to pass the salt?" (maybe the other person couldn't reach it!), although it is really being used to mean "Would you please pass the salt?". In this case, may is wrong because she is not asking or giving permission: she is making a request.

So:

may and can are used interchangeably when asking or giving permission.
would (or will) and can (or could) are used interchangeably when making a request.

[I was going to support this by referring to dictionary definitions, but @terdon's answer (with definitions) got posted while I was still writing mine, so I've omitted doing that.]

Solution 2:

There is nothing illogical about it at all. It is just misplaced in this particular case.

May I is asking for permission while can I is asking whether an action is possible. However, as you can see in the following definitions (both come from the online Merriam Webster, can and may), this distinction between can and may seems to be in the process of becoming obsolete:

can, transitive verb
1 a : know how to
b : be physically or mentally able to
c —used to indicate possibility ; sometimes used interchangeably with may
d : be permitted by conscience or feeling to
e : be made possible or probable by circumstances to
f : be inherently able or designed to
g : be logically or axiologically able to <2 + 2 can also be written 3 + 1> h : be enabled by law, agreement, or custom to

2 : have permission to —used interchangeably with may

may, verbal auxiliary
a archaic : have the ability to
b : have permission to : be free to —used nearly interchangeably with can
c —used to indicate possibility or probability ; sometimes used interchangeably with can ; sometimes used where might would be expected

This was always illustrated to me by my father with the following example, which I just found out is adapted from The Hickory Limb :

Mother, can I go swimming?

Yes, my darling daughter; Hang your clothes on a hickory limb.

And don't go near the water.

In the example above, the daughter of an admittedly annoyingly literal mother, is asking whether she is physically able to go swimming, not if she has the permission to do so and the mother answers accordingly.

In the case of your niece, if you wanted to be pedantic, you could say that she is asking her mother whether she—her mother—has permission to pass the salt, while what she wanted to do is to ask her mother to please pass the salt.

Solution 3:

My explanation would be that when she says, "May you please pass the salt?" she is saying "Do you 'have permission' to please pass the salt?" Instead she should be saying, "Would you please pass the salt." which means, " 'Are you willing (it is at your discretion) to please pass the salt?"