"Jsi dost stará na to, abys to věděla."

Translation: You are old enough to know that.

[pampelisky]

Why would abyste be wrong here? It gave me the option between abys and abyste, and I don't understand.

[kacenka9]

This sentence seems to be disabled for the czech side, so not sure where it popped up at you. Nevertheless, yes, abyste should be fine, but do not forget that in such a case, the first part of the sentence also has to be formal and use JSTE dost stará...

[CathyBowde1]

'jsi' is the informal singular you form, so you need the informal singular 'bys' to match it.

[BarbaraWhi574572]

Why is "na to" in the first clause of the sentence.
Would "Jsi dost stará, abys to věděla." be correct?

[kacenka9]

It is, more or less, an idiom. You can skip it but it is commonly used here. It means "for it". "You are old enough for it, so you should know better" would be a loose translation.

[teabeadle]

Would "you are old enough that you should know that" be a feasible translation here?

[uncle_tom]

I think so; I'm a native Polish and Czech learner, so don't take my words as gospel, but in Polish a very similar structure exists and yours would be a feasible translation.

[DaanBanaan18]

Very poetic-like structure to a foreign ear, but everyday use of the language for Czech. Interesting sentence.