A finite verb agrees with its subject in number and person.
Thus, τοῦτο τὸ ψήφισμα ἐγένετο
this bill was passed 13.56, ὃ δέδοικ᾽ ἐγὼ μὴ πάθηθ᾽ ῡ̔μεῖς
which I fear lest you may suffer 9.65, ἢν δ᾽ ἀποψηφίσωνται οἱ ἄλλοι, ἄπιμεν ἅπαντες τοὔμπαλιν but if the rest vote against (following), we shall all return back again A. 1.4.15, τὼ ξένω τώδε φίλω ἐστὸν ἐμώ
these two strangers are friends of mine G. 487a.
a. The verbal predicate, when a copulative verb (917), may be attracted to the number of a predicate noun, which often stands between subject and verb: τὸ χωρίον τοῦτο, ὅπερ πρότερον Ἐννέα ὁδοὶ ἐκαλοῦντο
this place which was formerly called Nine Ways 4.102, ἅπᾱν τὸ μέσον τῶν τειχῶν ἦσαν στάδιοι τρεῖς
the entire space between the walls was three stades A. 1.4.4. So with the participles of such copulative verbs: τὴν ἡδονὴν διώκετε ὡς ἀγαθὸν ὄν (for οὖσαν) you chase after pleasure as if it were a good Pr. 354c.