From Seinfeld

[S&S]

Elaine: Guess whose birthday's comin' up soon?

Jerry: I know, I'm having my root canal the same week.

Elaine: Oh, right. I hope you have a good oral surgeon because that ___(can or could) be very serious.

What does "can" indicate here?
What does "could" indicate here?

Thanks.

[Beeesneees]

That can be - in some cases it is...
That could be - conditional. in some situations it is...

Either would work there with no significant change of meaning. A native speaker would not consider differentiating between them in almost all cases.
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Cheers m' dears!

[S&S]

Then what about these two:

1. How can you be so stupid?
2. How could you be so stupid?

Could these two expressions be used with the same meaning in some situations?

By the way, I LOVE the way you handle all my questions. Many thanks!!! :-)

[Beeesneees]

Yes. Despite the difference in tenses which means that logically one is present and one is past, when this is used the stupidity has usually just been discovered. Either works for native speakers.

Are you some sort of a self-esteem guru or something, S&S? You always end your messages with such encouragement! :D
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Cheers m' dears!