μεγάλα ὠφελήσεσθε πρὸς ἱστορίαν τῶν κοινῶν → that will be of great benefit to you in order to understand public affairs
dat. de ὑμεῖς.
irregular dative case of ὑμεῖς; to (with or by) you: ye, you, your(-selves).
ὑμῖν: dat. к ὑμεῖς.
