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Is "could" interchangeable with "can"?

for example, there are coversations like these:

example. 1

A: I could run faster than you.
B: No way.

In this case, the "could" is referring to the less certain possibility of a skill in the present time.

example. 2

A: Even that small stadium could have ten thousand people.
B: Isn't that too small for ten thousand people?

In this case, the "could" is referring to the less certain possibility of a capability as a fact.

I think these "could"s in the two examples express the same things as "can", and the difference from" can" is just that "could" is less certain about possibilities.

However, I'm not sure if it's allowed to use "could" like those, because "could" is used when expressing hypothetical or conditional things like "would".
Are the "could"s acceptable?

Thank you.

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No. Could is defined as past of can. Could indicates possibility, while can means able to do so.

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    Another difference is that can implies absolute possibility but could implies a possibility which is conditional. EG "Usain Bolt can run faster than me but I could run faster than him if his legs were tied together". The same thing applies to the stadium: "That stadium can hold 5000 people" implies that there are 5000 seats whereas "That stadium could hold 10000 people" suggests that the capacity could be increased (perhaps by taking out the seats and making them stand). We do also use could for the past as you say: for example "A T-Rex could run faster than a brontosaurus". – BoldBen Dec 9 '17 at 8:17
  • @BoldBen Thank you! So when I want to show that I don't know why but I feel I can run faster than you, but it's just my guess, should I say either "Maybe I can run faster than you", "I think I can run faster than you, or "I may/might be able to run faster than you"? – Stud Dec 9 '17 at 8:52
  • @idiotprovider That's simple and easy to understand. Thank you. – Stud Dec 9 '17 at 9:01
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    Far too simplistic (and answers on ELU are almost always required to have supporting evidence). 'Could you open the door, John' and 'Can you open the door please, John' are virtually interchangeable polite requests. – Edwin Ashworth Dec 9 '17 at 11:09

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