Can is a statement of ability; may is a statement of permission or possibility. We all roll our eyes at the old joke in which a student says, “Can I use the bathroom?” and the teacher says, “I don’t know, can you?” but the joke holds because the usage holds. Try not to confuse the two words in formal writing. In some informal contexts, especially colloquial, proper usage might sound prissy, especially when the negative form is used. In such contexts, you may prefer to rely on can’t rather than mayn’t (rare) or may not. Why can’t he come? sounds more natural than Why mayn’t he come? or Why may he not come? even though the latter two are more correct.