We often use can in questions and negatives to talk about the logical possibility that something is true or that something is happening.
- I can hear a strange noise. What can it be?
- There’s the doorbell. Who can it be? Well, it can’t be the milkman. He has already been.
We do not normally use can in affirmative sentences to talk about logical possibility. This meaning is expressed with could, may or might.
- ‘Where is Ramya?’ ‘She could / may / might be at the library.’ (NOT She can be at the library.)
- Where can he have gone? He can’t have gone to office. It is Sunday. He can’t have gone to the temple either. He never goes there. I think he could / may / might have gone to the market. (NOT I think he can have gone to the market.)
Asking for and giving permission
We use can to ask for and give permission. Cannot (can’t) is used to refuse permission.
- ‘Can I use your computer?’ ‘Yes, of course you can.’
- ‘Can I borrow your car?’ ‘No, I’m afraid you can’t.’
- ‘Can I go now?’ ‘No, you can’t.’
- ‘Can I have an ice cream?’ ‘Of course you can.’
- ‘Can I ask you something?’ ‘Yes, you can.’
We can also use could to ask for permission. It is more polite than can. Note that could is not used to give or refuse permission.
- ‘Could I ask you something?’ ‘Of course you can.’ (NOT Of course you could.)
- ‘Could I go to the movies, Mom?’ ‘No, you can’t. (NOT No, you couldn’t.)