"Can" expresses ability or capacity (to be able to do something); "may" expresses formal permission or possibility (something might happen).
Modal verb: (1) Ability or capacity: "She can swim" = she knows how to swim. "I can speak three languages" = I am able to speak three languages. (2) Informal permission: "Can I go now?" = am I allowed to leave? (informal). (3) General possibility: "It can get very cold in December" = it is possible for it to get very cold. In everyday English, "can" has largely replaced "may" for asking permission.
She can play the piano beautifully — she has been taking lessons since she was four years old.
Can you help me move this table? It is heavier than it looks.
The traffic on that road can be terrible on Friday afternoons — I suggest taking the highway instead.
Modal verb: (1) Formal permission: "May I come in?" = am I permitted to enter? (formal). "You may leave the exam room" = you are permitted to leave. (2) Possibility: "It may rain tomorrow" = there is a possibility it will rain. "She may be late" = it is possible she will be late. In formal and written English, "may" is preferred for permission. In everyday speech, "can" has nearly replaced it.
May I ask what time the meeting is scheduled for? I want to make sure I am not late.
The patient may experience some mild discomfort after the procedure, but it should pass within 24 hours.
You may use the conference room until 3 p.m., but it will be needed for a client meeting after that.
Common mistake
The most famous prescriptivist debate in school grammar: traditional teachers insisted "can" was for ABILITY and "may" was for PERMISSION. The classic response: "Can I go to the bathroom?" — teacher: "I don't know, CAN you?" (meaning: "do you have the physical ability to go to the bathroom?"). In real modern English, "can" for permission is completely accepted even in formal contexts. The strict can/may distinction for permission is a 19th-century prescriptivist rule that does not reflect current usage. For POSSIBILITY, "may" remains more precise than "can".
Trick to remember it
Modern practical rule: CAN = ability ("I am able to / I know how to"). MAY = possibility ("it is possible that") or VERY formal permission. For everyday permission requests = CAN or MAY, both accepted. For future possibility = MAY (or "might" for more doubtful possibility). "It can rain tomorrow" = rain is generally possible (capacity). "It may rain tomorrow" = there is a specific possibility of rain tomorrow (more targeted). Difference between MAY and MIGHT: may = higher possibility (50-60%), might = lower possibility (30-40%).
Quick rule
Can = ability or capacity (knowing how to do something) and informal permission. May = future possibility ("it is possible that") and formal or written permission. In everyday speech, can for permission is completely valid.
Did you know?
The correction "do not say can, say may for permission" was popularized by 19th-century prescriptivist grammar books, especially by the manuals of Lindley Murray (1745-1826), whose grammar was the best-selling textbook in the history of English for over 100 years. Murray, an American Quaker lawyer who retired to York (England), wrote his grammar rules based on Latin logic rather than actual English usage. His influence was so massive that his rules continue to appear in textbooks nearly 200 years later.
Exercise 1 · Pick the right one
She _____ speak four languages fluently — her parents raised her in a trilingual household and she learned Japanese in university.
Exercise 2 · Pick the right one
The project timeline _____ change depending on the budget approval — we will know more by Friday.
Exercise 3 · Form the word
Can = ability or capacity (knowing how to do something) and informal permission. May = future possibility ("it is possible that") and formal or written permission. In everyday speech, can for permission is completely valid.
The most famous prescriptivist debate in school grammar: traditional teachers insisted "can" was for ABILITY and "may" was for PERMISSION. The classic response: "Can I go to the bathroom?" — teacher: "I don't know, CAN you?" (meaning: "do you have the physical ability to go to the bathroom?"). In real modern English, "can" for permission is completely accepted even in formal contexts. The strict can/may distinction for permission is a 19th-century prescriptivist rule that does not reflect current usage. For POSSIBILITY, "may" remains more precise than "can".
Modern practical rule: CAN = ability ("I am able to / I know how to"). MAY = possibility ("it is possible that") or VERY formal permission. For everyday permission requests = CAN or MAY, both accepted. For future possibility = MAY (or "might" for more doubtful possibility). "It can rain tomorrow" = rain is generally possible (capacity). "It may rain tomorrow" = there is a specific possibility of rain tomorrow (more targeted). Difference between MAY and MIGHT: may = higher possibility (50-60%), might = lower possibility (30-40%).
Bryan López
Founder of SpeakUP Academy