649 Εἰ with the indicative imperfect and aorist, rarely the pluperfect, is used to imply that the supposition is past realization, opposed to a known fact (Unreal Past Condition).

The imperfect is used when the reality would be expressed by the present or imperfect, the aorist when the reality would be expressed by the aorist, the pluperfect when the reality would be expressed by the perfect:

Φῶς εἰ μὴ εἴχομεν, ὅμοιοι τοῖς τυφλοῖς ἂν ἦμεν if we did not have light (as we do) we should be like the blind. M. IV. 3, 3. οὐκ ἂν νήσων ἐκράτει, εἰ μή τι καὶ ναυτικὸν εἶχε he would not have ruled οver islands, if he had not had some naval force (therefore he had, εἶχε ναυτικόν). T. I. 9.

Eἱ μὴ μεῖς ἤλθετε ἐπορευόμεθα ἂν ἐπὶ βασιλέᾱ if you had not come(but you did, ἤλθετε), we should now be marching against the king. AN. II. 1, 4. εἴ τίς σε ἤρετο, τί ἂν ἀπεκρνω; if some one had asked yοu (nobody did), what shοuld you have answered? PR. 311 b. εἴ σε τύχη προύπεμψε καὶ ἡλικίᾱς ἐπέβησεν, ἐλπίδι γʼ ἦσθα μέγας if fortune had guided thee on and brought thee to manhood, in hope at least thοu wast great. CIA. 2263.

Εἰ ἐκεκτήμην οὐσίᾱν, ἐπʼ ἀστράβης ἂν ὠχούμην if I possessed property, I should ride on a saddle. L. 24, 11.