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One person here explained me the difference between can and could

"Could indicates that something is possible, but not certain. Can is used in general statements about what is possible.

So, you may use 'can' to speak about the weather in general, such as "it can rain a lot in winter*" or "it can be very dry during the summer". However, your example is about a specific forecast - tomorrow - so it isn't a generalisation."

I still have one issue related to questions my book says that this example is fine "who can be knocking on the door?". Why is it correct. It's not a generalisation plus it's not about abilties but rather likelihood?

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    Yes, another use of can is to wonder about who/what an unidentified person or thing is. "Who can that be, knocking on the door at this time of night?" "This is a mysterious parcel. What can it be?" Mar 25 at 9:30
  • Are we allowed to use "may" in that question? Mar 25 at 10:19
  • You are not forbidden to (!) but it wouldn't sound natural. Mar 25 at 10:43
  • You can also say "who could it be knocking on the door?"
    – Stuart F
    Mar 25 at 10:48
  • It absolutely is a question about likelihood, or possibility. "Who could be knocking at the door" is the idiomatic choice.
    – Astralbee
    Mar 25 at 11:20

1 Answer 1

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Statement: Could indicates that something is possible, but not certain. Can is used in general statements about what is possible.

Can [present tense] you swim now because before you coudn't. [past tense]

Couldn't he go because he was sick? [past] Can he go because he isn't sick? [present]

In conditionals: I can go, if you want me to. [possible that I go] Can I go, if you want me too?

I could go, if you wanted me to. [less possible that I go] Could I go if you wanted me to?

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