A.with a verb in protasis, answered by a similar tense in apodosis:
1.with pres. and fut. ind., to express mere Possibility, εἰ τοῦτο ποιεῖ (or ποιήσει), ἁμαρτάνει (or ἁμαρτήσεται) if he is doing (or shall do) this, he is (or will be) wrong.
2.with imperf. and aor. ind., to express Impossibility, εἰ τοῦτο ἐποίει, ἡμάρτανεν ἄν if he was doing this, he would be wrong; εἰ τοῦτο ἐποίησεν, ἥμαρτεν ἄν if he did (or had done) this, he would be wrong.
3.with opt. to express a mere Assumption, εἰ τοῦτο ποιοῖ, ἁμαρτάνοι ἄν if he were to do this, he would be wrong.
4.with Subjunctive, to express Possibility with some degree of Probability: in this case ἄν is always added, and εἰ ἄν becomes ἐάν, ἤν, ἄν, (epic εἴ κεν), ἐὰν τοῦτο ποιῇ, ἁμαρτήσεται if he do this, he will be wrong.
II.sometimes the apodosis is omitted, so that εἰ expresses a wish, εἴ μοι γένοιτο φθόγγος if I had a voice, [I would . . ], i. e. Oh that I had a voice! so εἰ γάρ, εἴθε, epic αἰ γάρ, αἴθε.
2.sometimes the protasis is omitted, εἰ δ᾽ ἄγε come on, = εἰ δὲ [βούλει], ἄγε, Od.; εἰ δέ, σὺ μὲν ἄκουσον Il.
B.In Indirect Questions, whether, Lat. an, followed by the ind., subj., or opt., according to the principles of oratio obliqua:
1.with INDIC. or SUBJ. after primary tenses, οὐκ οἶδ᾽, εἰ θεός ἐστιν whether he is a god, Il.; οὐκ οἶδ᾽ εἰ δῶ whether I shall give, Xen.
