This is a tough one!
Ability or Permission?
There was once a time in the land of Strict Grammar Rules when “can” suggested physical or mental
ability and “may” denoted permission
or authorisation.
It WAS NOT acceptable to use “can” if you were talking about
permission. You could hear citizens of this land saying, “May I accompany you
to the ball, Miss Ruby?” and “Why of course you may, my dear.” This young lady would
perhaps ask her suitor about his dancing ability: “Can you do the cha-cha?” and
he would answer that he did have the ability: “Why of course I can, Miss Ruby.”
“Can” Instead of Traditional “May”
Nowadays, the rules aren’t so cut and dried! Since the second half
of the 19th century, “can” has been used in informal contexts to denote
permission.
You’ve probably heard someone ask, “Can I go to the party?” If we
lived in strict-grammar land, the authorities would complain about this
usage, but these days it is acceptable to use “can” in this manner if you’re
speaking informally.
If you’re a teacher of young children, you probably often hear “Can
I go to the bathroom?” Parents probably hear their children whining, “Can I
have a cell phone?” So are the kids to blame for using “can” instead of “may”?
Well, they just repeat what grown-ups say, and grown-ups are apparently moving
away from the sometimes prissy-sounding “may.”
“May” Is OK
Nowadays, “may” does have its rare place. If formality and
politeness are of utmost importance, you should use “may” to denote permission.
So it would be better to say to the waiter at a fancy restaurant, “May I have
more water, please?” than “Can I have more water, please?” If you’ve just
knocked on a door, you should probably say, “May I come in?”
Hope this clears the confusion.
