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" 4 could, might, may, etc. do something yet used to say that something could, might, etc. happen in the future, even though it seems unlikelyWe may win yet. (formal) She could yet surprise us all.

Advanced Oxford Dictionary "

I can't understand when it is said formal (She could yet surprise us all.)

Is could more formal than may?

I would be grateful for your help.
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I believe it means that definition "4" is a formal usage of some word whose identity you have kept secret.

No distinction is intended among "could," "might," and "may."

Okay, the definition is talking about the expression "She could yet surprise us all," which is only used formally.

In that expression, you may substitute "might,may,etc." for "could."
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JesusenglandI can't understand when it is said formal (She could yet surprise us all.)
Is could more formal than may?
That dictionary claims that yet makes it formal, not may, might, or could. It has nothing to do with the relative formality of may, might, and could.

(Personally, I don't see anything particularly formal about yet, but then my personal opinion is not the point here.)

CJ