Can, Could, May & Might: Ability and Permission

Level B1 · Modals

Can, could and may cover two separate jobs here — ability (what someone is capable of doing) and permission (what someone is allowed to do). Ability first:

• can = general present ability: I can swim; she can speak Japanese.
• could = general PAST ability, something you were able to do repeatedly or as a skill over a period: When I was a child, I could climb trees easily. She could speak three languages by the time she was ten.
• could is NOT used for a specific one-time success in the past — for a single achievement, use was/were able to or managed to instead: Despite the flat tyre, she was able to finish the marathon. After three attempts, he finally managed to open the jar.
• will be able to = future ability: I'll be able to drive once I pass my test. You won't be able to enter without a ticket.

Permission works differently, and the choice between can, could and may is mostly about formality:

• can = informal permission, everyday requests and grants: Can I use your phone? You can sit here if you like.
• could = a more polite, slightly more formal request for permission: Could I leave early today? Could we open the window?
• may = the most formal and polite way both to ASK for permission and to GRANT it: May I come in? — Yes, you may. May I ask you a question? You may leave now.

The most common mistake is using could for a specific one-time past achievement: I could win the race yesterday is wrong. Because it describes a single successful event, it should be I was able to win the race yesterday or I managed to win the race yesterday — could works only for general, repeated, or ongoing ability, not for one particular success. Note the exception: could DOES work for a one-time failure or a negative statement in the past, so I couldn't find my keys yesterday is perfectly fine — the restriction is specifically about one-time POSITIVE achievements. A second common mistake is over-using may in casual situations: May I go to the bathroom? suits a formal classroom, but May I borrow a pen from my friend? sounds oddly stiff between friends — can or could fit that context much better. Finally, don't confuse this permission may with the may that expresses possibility (it may rain later) — that is a completely different use, covered elsewhere.

Look for general or habitual past-time signals — when I was young, in those days, every summer, by the time she was ten — these point to could for ability. Look for single-event signals — despite, finally, after several attempts, somehow, yesterday's specific race — these point to was/were able to or managed to instead. And whenever you see a request or a grant of permission (can/could/may + I/we + base verb), judge the right modal from the stated formality of the situation: a close friend suggests can or could, a stranger, a boss, or a formal letter suggests could or may.

Practice

Ability: General Past vs One-Time Success · Permission: Can, Could & May