Many people still are not able to use these words correctly. They may think that both of them have the same function whether it is used for asking/giving permission or showing ability to do something. In fact, the function of both “can” and “may” are not the same. After reading this article, you will understand and take a self-correction about those two words used.
Have you ever said, “Can I borrow your book, please?” to someone to ask a permission? How do you think? Was it correct? No. I don’t think so. Why is it so? Because the use of “can” is not for asking or giving a permission, but it is used for showing ability to do something. Therefore, the example above is not correct yet. The correct one is “May I borrow you book, please?”. Moreover, “Can” is used to show an ability of yourself or someone else. The appropriate example of “can” is “Can you speak Malay?” or “I can do this better than anyone else”.
How about “May she swim?” sentence? Is it true? Is it appropriate? No, it is not. why? Because it should be showing an ability, so you do not need to use “may” but “can”. Hence, the correct one is “Can she swim?”.
Actually, you can also use “can” to express permission but that is only for informal situation or style. Thus, it is not appropriate to say “Can I go to the restroom, Ma’am?” to your teacher because it is impolite. You would better say “May I go to the restroom, Ma’am?”. So far, do you understand?
It seems not so important to know, but I am sure that it is not as trivial as you think because it deals with the culture. You will get a serious problem if you only learn a language without knowing its culture. People will respect to someone who knows how to place his/herself in a right place. Have a great day! :D