Can, could or may ? - gramática inglés en "English Grammar Today" - Cambridge University Press
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Can, could or may?

from English Grammar Today

Possibility

When we talk about possibility, we use can, could and may, but they are different in meaning.

Compare

It can be dangerous to cycle in the city.

This expresses what the speaker believes is a general truth or known fact, or a strong possibility.

It could/may be dangerous to cycle in the city.

This does not express a general truth. The speaker is only expressing a weak possibility.

It’s dangerous to cycle in the city.

This expresses a real fact using the present simple. The speaker is certain and no modality is used.

Permission

We use can, could and may to ask for permission. We use can and may, but not could, to give permission. May is less common:

Compare

asking for permission

formal/polite?

giving permission

formal/polite?

Can I ask you a question?

informal

Yes, you can.

Yes, you may.

Yes, you could.

informal

Could I use your phone?

more formal/polite

formal/polite

May I use your phone?

even more formal/polite

Requests

When we make requests, we can use can or could (but not may). Could is more polite than can:

Can you call back later? I’m busy now.

Could you call back later? I’m busy now.

Teachers and parents often use can and could in requests:

Can you open you books at page 34, please.

Can you please refrain from chewing gum.

Could you just sit down and listen!

Can, could or may: typical errors

  • Could in the present only expresses weak possibility. Can expresses strong possibility:

I can travel in July because my exams will definitely be finished at the beginning of that month. (strong possibility)

I could travel in July because my exams will probably be finished at the beginning of that month. (weak possibility)

  • We don’t normally use could to talk about general truths which refer to the present time. We use can instead:

Not everyone can afford to buy organic food.

Not: Not everyone could afford

  • We use could, not can, to express future possibility. Can expresses that we are certain of something:

Working in London next summer could be a great experience. (The speaker thinks this is possible, in particular circumstances)

Not: … can be a great experience.

(“Can, could or may ?” from English Grammar Today © Cambridge University Press.)
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