See a brief explanation of the basic structure of all modal auxiliary verbs...
The question forms of these modal auxiliaries (and most other modals) are formed by inverting the subject and the modal. The negative forms take not. Also see the possible contracted forms. The main verb is always in infinitive without to:
You can swim. - Can you swim?
Mary could finish the report. - Could Mary finish the report?
We may go home now. - May we go home now?
It might rain later. - Might it rain later?
You cannot* swim. (You can't swim.)
Mary could not finish the report. (Mary couldn't finish... .)
We may not go home. (We mayn't** go home.)
It might not rain. (It mightn't rain.)
Short answers are:
Yes, I can. / No, I can't OR No, I cannot.
Yes, I could. / No, I couldn't.
Yes, you may. / No, you may not.**
Yes, he might. / No, he mightn't.
*Can not is not possible.
**Mayn't exists but is rare.
We use can, could, may and might to ask for permission. It's probably true to say that could and may are more polite than can but tone of voice is also very important:
Can I have another biscuit, please?
Could I finish this report tomorrow? I'm feeling tired.
May I borrow your pen for just a moment?
When we use might to ask permission, we can sound timid, cautious or even nervous:
Might I ask you a personal question?
In a similar way, we can offer to do things with these auxiliaries:
Can I help you with those heavy bags?
Could I offer you a piece of chocolate cake?
May we drive you back to your house, Mrs Jones?
Compare with the use of would for being polite...
We use especially can and could to ask for things (advice, help, etc.) from other people. However, we cannot use may with the second person you in questions to ask for things:
Can / Could / May you tell me the way to the train station?
Can / Could / May you give me a hand?
We use can and could (but not may and might) to talk about ability. This could refer to a skill or if the circumstances (availability) allow us to do something. Compare:
Can you play football? (This means: have you leant how to play?)
and,
Can / Could you play football this afternoon? (This means, for example, do you have time to play? - circumstances, are you available?)
More examples:
Can / Could you come to dinner tonight? (circumstances)
Can you do this homework? I can't. (ability)
Can you speak Chinese? (ability)
However, could is not used to talk about skills in the present; we use could to talk about skills and circumstances in the past or in the conditional:
I could speak Chinese when I was a child (past tense).
I couldn't do my homework last night. It was too difficult (past tense).
I could speak Chinese if I had a good teacher (conditional).
If I won the lottery, I could buy a yacht (conditional).
However, we cannot use could to say: I could fix the car yesterday. Learn why below...
We use can to talk about possibilities in a present simple way (often, sometimes, usually, etc):
Spain can often be very hot in summer.
He can be very bossy sometimes.
I can speak better English when I'm at parties (always).
For future possibilities, we must use: may, could or might but not can:
It may / might / could / can rain tomorrow.
She's not in. She may / might / could / can come back this evening.
We use may and might to talk about possible plans we have for the future:
We may go to Ibiza next year.
I might play tennis tomorrow though it depends on how I feel.
When we want to deduce (or guess) what something is or what is happening or what happened, we use may, could or might. The difference in meaning between these words is the degree of possibility. May expresses more possibility, could, less possibility and might, very little possibility:
I don't know what it could be, I think it may be a fried egg or it might be the sun but I don't think so (when looking at a picture).
Where's my watch? Could it be at home?
My friend may be about 32 I think.
What might this be?
Where could he have got to?
Can is used in this context of deductions but never in the affirmative; only in questions or negative sentences:
Who can that be arriving in a car?
It can / could / may / might be Jim.
That can't possibly be right.
Yes, it can / could / may / might be right.
No, it can't be a fried egg; it isn't yellow.
Modal auxiliaries must, can't, may, could, might for deductions (listen and read)...
If we are making a deduction about something that happened in the past, we use the structure: modal auxiliary + have + past participle:
(Arriving at a friend's house) Look, John's car isn't there. Where could he have gone? He could have gone to the cinema or he may have just gone out for a few minutes. He might have gone on holiday but I don't think so.
Hey, look! He can't have gone out for very long; the front door is open!
When we talk about a finished past actions which happened at specific times in the past, we cannot use could in affirmative sentences. We must use alternative verbs: be able to, manage to, succeeded in:
I could was able to speak to your teacher this afternoon.
I could managed to fix the car in the end.
They could finish succeeded in finishing all their work before six o'clock.
But we can use could in questions and negatives for past finished actions:
I've left my car in the car park because I couldn't find my key.
Despite its agility the deer couldn't (wasn't able to) escape the jaws of the tiger.
Could you fix the car?
Listen and read to my podcast on ability in the past...
Exercises on can, could, may and might...
Explanation of modal auxiliary verbs in general...
Modal auxiliary verbs 2 (should, ought to)...
Modal auxiliary verbs 3 (must,)...
Modal auxiliary verbs 4 (would)...
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