(I) πρίν takes the subjunctive with ἄν to denote a future action anticipated by the subject of the leading verb. The principal clause is negative, and contains any verb referring to the future except the simple optative.
οὐ πρότερον κακῶν παύσονται αἱ πόλεις πρὶν ἂν ἐν αὐταῖς οἱ φιλόσοφοι ἄρξωσιν
μηδένα φίλον ποιοῦ πρὶν ἂν ἐξετάσῃς πῶς κέχρηται τοῖς πρότερον φίλοις
a. The aorist subjunctive is usual (the tense of negation, 2439, and of action prior to that of the principal clause); much less common is the present subjunctive (usually of contemporaneous, overlapping action) as
b. Homer does not use κέ or ἄν in this construction since πρίν is here adverbial and its clause is simply coördinated. But Hom. has πρίν γ' ὅτ' ἄν. The subjunctive without ἄν occurs occasionally as an archaism in Hdt. and the dramatic poets. In Attic prose especially in Thuc. (e.g. 6. 10, 29, cross38); but ἄν is often inserted by editors.
c. The leading verb is rarely the optative with ἄν (as a form of future expression): οὐκ ἂν ἀπέλθοιμι πρὶν παντάπα_σιν ἡ ἀγορὰ_ λυθῇ (cp. b)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].