<em>Can</em> is mainly about ability: 'She can swim a mile.' <em>May</em> is about permission or possibility: 'You may leave when you're finished' and 'It may rain later.' The two overlap on permission, where <em>can</em> is normal in everyday speech and <em>may</em> is the formal choice.
The core difference
Strip away the folklore and there are three jobs here:
- Ability — only can. 'He can read Japanese.'
- Permission — both. 'Can I borrow this?' / 'May I borrow this?'
- Possibility — both, but differently. 'It may snow' means there is a real chance. 'It can snow in April' means snow in April is a thing that happens sometimes.
The test
Try substituting 'am able to'. If it fits, you need can: 'I am able to lift it' → 'I can lift it.'
Try 'am allowed to'. If that fits, you are in permission territory, where either word is grammatical — choose by register. Formal writing, a request to a stranger, a legal or institutional context: use may. Anything else: can is fine.
Being honest about the permission rule
The schoolroom correction — 'Can I go to the toilet?' 'I don't know, can you?' — is a joke about a rule that ordinary English does not follow. Using can to ask permission has been standard for a very long time and appears throughout respected writing. Insisting on may in casual speech marks you as fussy, not correct.
Where may genuinely earns its place is formality and politeness. 'May I ask what this is regarding?' is warmer and more deferential than the same question with can. In contracts, notices and instructions, may is also the conventional way to grant a right, while must imposes a duty — a distinction lawyers take seriously.
The possibility difference is real
This is where confusing the two actually changes meaning. 'This medicine may cause drowsiness' says it might do so on any given occasion. 'This medicine can cause drowsiness' says it has that capacity in general. In safety and technical writing, pick deliberately.
Negatives and the past
Cannot and may not are not equivalent. 'You cannot park here' states an impossibility or prohibition; 'You may not park here' is purely a prohibition. Meanwhile 'It may not rain' means rain is possible but not certain — a much weaker statement than 'It cannot rain'.
For the past, can becomes could. May has might, which in practice most writers now use for possibility in both present and past.
Frequently Asked Questions
Is it 'Can I ask you something?' or 'May I ask you something?'
Both are correct. 'Can I' is what most people say and is entirely standard; 'May I' is more formal and polite, and is the better choice with someone senior or unfamiliar.
Is it 'may of' or 'may have'?
Always 'may have'. 'May of' is a mis-hearing of the contraction 'may've' and is never correct in writing.
Which should I use in instructions or policies?
Use 'may' to grant permission, 'must' for requirements and 'cannot' for things that are impossible. Mixing 'can' into a policy document blurs permission with capability.