I am teaching modal verbs to my class. I am having a problem using modal verbs of permission. The modal verbs "can" and "may" are used to show permission. My problem is "Can these two words be used interchangably?" Or are there specific rules governing each?
Eg: May I borrow your pen please?
Can I come in?
May I have a glass of water please?
Can I have a glass of water please?