As I child, did you ever ask your teacher, “Can I go to the bathroom?” only to be corrected “I don’t know, can you?” You’d look confusedly at her, and she’d say, “It’s may I go the bathroom.” You grumbled “May I go the bathroom?” and with hall pass in hand head off to do your business.

Yet somehow that lesson didn’t seem to sink in for a lot of writers, who still confuse can and may in adulthood.

Can is a helping verb that means physical or mental ability; to wit, Can Kieran play basketball really well? A form of able or capable always can replace can in a sentence, as in Is Kieran capable of playing basketball really well?

May is a helping verb that requests permission, as in May I go with Mary to the concert, Mom? Or suggests the possibility of, as in Tomorrow may be a snow day.

This distinction between can and may only exists when asking a question. If answering a may question (May I turn the channel? ) with a “no” or a denial, you can respond with a version of can:
• You may not turn the channel.
• You cannot turn the channel
• You can’t turn the channel.

Of course, may and can slowly are becoming interchangeable in daily use. A century from now (and maybe sooner), the distinctions outlined here may be archaic. For the time being, when in a formal situation or being polite, always use the grammatically correct version described here.