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 colloquial situations, proper usage might sound prissy, especially when using the negative forms. In such contexts, you may prefer to rely on can’t over 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 when the question is one of permission.