Can vs. Could

• Glass can be blown.
• Can gases freeze?

We use could to talk about past possibility.

• It was a place where anything could happen.

Cases where can is not used

We do not use can to talk about the chances that something will happen in future. We express this idea with may, might or could. Note that could suggests a less definite possibility than that is implied by may or might.

Compare:

• There may/might be a strike next week. (Future probability) (NOT There can be a strike —)
• Strikes can happen any time. (Theoretical possibility)
• It could rain later this evening. (Less definite future possibility) (NOT It can rain later this evening.)

Cases where can is used

We use can in questions and negative sentences, to talk about the logical possibility that something is true or that something is happening.

• ‘There is the doorbell.’ ‘Who can it be?’ ‘Well, it can’t be the postman. He has already been.’

Note that can is not usually possible in affirmative sentences with this meaning. Instead, we use could, may or might.

• ‘Where is Jane?’ ‘She could/may/might be at Joe’s place.’ (NOT She can be –)

But can is possible in affirmative sentences with words like only, hardly, never etc. Can only means must.

• ‘Who can it be at the door?’ ‘It can only be the postman.’ (= It must be the postman.)