324*. We may notice here the cases in which εἰ with an indicative or subjunctive is not followed by a corresponding clause expressing the consequence of the supposition made. This occurs
a. When two alternative suppositions are made, the second being the one upon which the speaker wishes to dwell.
Il. 1.135-7 εἰ μὲν δώσουσι γέρας . . .
εἰ δέ κε μὴ δώωσιν, ἐγὼ δέ κεν αὐτὸς ἕλωμαι
if they give (there is nothing to be said), but if not, etc.
b. When the consequence is sufficiently implied in the εἰ clause.
Il. 6.150 εἰ δʼ ἐθέλεις καὶ ταῦτα δαήμεναι
if you wish to be tοld this (I will do so)
Il. 7.375 αἴ κʼ ἐθέλωσι
if they wish to cease (let them)
Od. 21.260 ἀτὰρ πελέκεας γε καὶ εἴ κʼ εἰῶμεν ἅπαντας
Il. 19.147, 20.213, 21.487, Od. 4.388, 15.80.
c. When the speaker prefers to suggest the consequence in an indirect way.
Il. 1.580 εἴ περ γάρ κʼ ἐθέλῃσιν Ὀλύμπιος ἀστεροπητὴς
ἐξ ἑδέων στυφελίξαι, ὁ γὰρ πολὺ φέρτατός ἐστιν
if he wishes (he will), for he is strong enough
Il. 14.331, 21.567, Od. 3.324.
There is a similar omission of the apodosis in causal clauses with ἐπεί at the beginning of a speech.
Il. 3.59 Ἕκτορ, ἐπεί με κατʼ αἶσαν ἐνείκεσας
ll. 6.382 Ἕκτορ, ἐπεὶ μάλʼ ἄνωγας κτλ.
Il. 13.68 & 775, Od. 1.231, 3.103 & 211. The full form appears in
Il. 6.333 ἐπεί με κατʼ αἶσαν ἐνείκεσας . . .
τοὔνεκά τοι ἐρέω.
In such sentences as εἰ δʼ ἐθέλεις . . . δαήμεναι some commentators obtain an apodosis by taking the infinitive as equivalent to an imperative, "if you wish, then learn, etc." But this is exceedingly forced, and indeed impossible in some places, e. g. Il. 7.375, Od. 21.260. Elsewhere the apodosis is forgotten (anacoluthοn); so after εἰ in ll. 22.111, after ἐπεί in Il. 18.101; Od. 4.204, 6.187 & 262, 8.236, 17.185.
Note— The omission of the principle verb in passages such as those in b and c above (especially when it is suggested by an infinitive in the protasis) finds a perfect parallel in the Law of Gortyn
iii.37 κόμιστρα αἴ κα λῇ δόμεν ἀνὴρ ἢ ϝῆμα δυώδεκα
στατῆρανς ἢ δυώδεκα στατήρων χρῆος, πλῖον δὲ μή
if man or wife choose to give payment for nurture,
let him or her give a garment or twelve staters
or something of the value of twelve staters, but not more
cp. the other places quoted by Baunack, Die Inschrift von Gortyn, p. 77. This shows that the usage must have been well established in Greek prose from an early period.