Can vs. could

[Rickyrocky]

Hi,

How could you tell if she's happy or not?
vs.
How can you tell if she's happy or not?

Can we use could in the present tense in this context?
What's the difference between them?

---
Do you want to bet how long their relationship would last?
vs.
Do you want to bet how long their relationship will last?

Which is correct and why?

Thanks.

[Bookaholic_English]

I think both are correct considering grammar. Could and would are more gentle as they express a little bit uncertainty while can and will more certainty and decisiveness. All four could be used to express a present time.
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