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