can / may
Ability vs. permission.
Can marks ability: She can lift the crate. May marks permission or possibility: You may leave. It may rain. In everyday speech, many people use can for permission, as in Can I leave early? In careful English, use may for permission: May I leave early?
Examples
UseThe lock can be picked. (ability)
UseYou may pick the lock. (if you are giving permission)
AvoidYou can pick the lock. (in careful English, if you mean permission)
Dictionaries list can for permission, and this use is standard in casual speech. However, many people still expect may in formal settings, and they judge the speaker who uses can for permission.