Have you heard this question before (perhaps when someone comes to your home or office): Can I sit down? What does it mean?
“Can” is about ability or capacity. So when someone asks “can I sit down”, that person is asking a stupid question. Why? Why would someone ask himself whether or not he has the capacity to sit down (¿por qué alguien se preguntaria si el personalmente tiene la capacidad de sentarse?)? He should know the answer.
So the question is a stupid one. Can is about ability to do things: can Ricardo cook well, can Ricardo play the piano? Many people use “can” when they should use “may”.
“May I sit down” is the correct question, the correct way to ask for permission. When mortals use “can I sit down” we understand (perhaps) what they mean, given the context, but it is an error, and a common error.
So to ask for permission use “may.