I’ll tell you outright that you gave wrong grammar advice to your Mom and have been wrong all along in using the construction “may you please.” It’s definitely bad form and terribly unidiomatic to use the modal “may” in tandem with the function word “please,” as in, say, the impossibly wrongheaded “
May you please open the door?”
The problem with “may you please” is not only that it’s grammatically redundant, “may” being a modal for humbly asking permission to do something and “please” being a function word for also expressing politeness when making a request. The bigger problem is that “may you please” actually breaches the upper limits of expressing politeness, making the speaker sound overly deferential and even fawning to the person being addressed. (To get a petition granted, imagine prostrating yourself before royalty in utter supplication.)
Remember now that in the hierarchy of English modals for asking permission, “may” is already the lowest in the totem pole for forthrightness in making a request, way, way below the modals “would,” “could,” “will,” and “can” (in that order of decreasing degree of forthrightness). Indeed, “may” is already at rock bottom for forthrightness and so exceedingly high for politeness that it can no longer take “please” to further intensify it.
Thus, while good English can’t countenance and has effectively banished constructions like “
May you please open the door?”, it gives wide latitude to the other request modals to work in tandem with “please,” allowing us to say “
Would you please open the door”, “
Could you please open the door”, “
Will you please open the door,” or “
Can you please open the door” depending on our desired degree of politeness or that socially required by the situation.
