εἰ μέλλω σοι ἕπεσθαι.
is this a conditional sentence or something else?
Yes, a favourite of Plato’s. See earlier at 324d: “πότερον ἔστιν τι ἓν ἢ οὐκ ἔστιν οὗ ἀναγκαῖον πάντας τοὺς πολίτας μετέχειν, εἴπερ μέλλει πόλις εἶναι;” (“is there some one thing–or not?–in which it is necessary for all the citizens to share, if indeed there is to be a city [sc. at all]”). The one at 334d goes like: “so cut up your responses and make them smaller, if I am to follow you”.