When a supposed future case is stated less distinctly and
vividly than the subjunctive would state it (as if
I should go in English), the protasis takes the optative with
εἰ. The apodosis takes
the optative with ἄν to
denote what would be the result if the
condition of the protasis should be fulfilled. E.g. Εἰ ἔλθοι, πάντ’ ἂν ἴδοι,
Εἴης φορητὸς οὐκ ἂν, εἰ πράσσοις καλῶς,
you would not be bearable if you should ever
be in prosperity.
This form of the conditional sentence in its fully developed use, as it appears in Attic Greek, must be carefully distinguished from that of 410; the more so, as we often translate both εἴη ἄν and ἦν ἄν by the same English expression, it would be; although the latter implies that the supposition of the protasis is a false one, while the former implies no opinion of the speaker as to the truth of the supposition. We have seen (438-440) that the more primitive Homeric language had not yet fully separated these two constructions, and still used the optative in the apodosis of present, and sometimes of past, unreal conditions.
On the other hand, the distinction between this form and that of 444 is less marked, and it is sometimes of slight importance which of the two is used. As it is often nearly indifferent in English whether we say if we shall go (or if we go) it will be well, or if we should go it would be well, so may it be in Greek whether we say ἐὰν ἔλθωμεν καλῶς ἕξει or εἰ ἔλθοιμεν καλῶς ἂν ἔχοι. In writing Greek, this distinction can generally be made by first observing the form of the apodosis in English; if that is expressed by should or would, it is to be translated by the Greek optative with ἄν; if it is expressed by shall or will, by the future indicative. Other forms of the apodosis, as the imperative, will present no difficulty. The form to be used in the protasis will then appear from the principles of the dependence of moods (170-178); the optative will require another optative with εἰ in the dependent protasis, while the future indicative or any other primary form will require a subjunctive with ἐάν or a future indicative with εἰ.457
In indirect discourse after past tenses we often find an optative in protasis, which merely represents the same tense of the subjunctive or indicative in the direct discourse. crossSee 667, 1; cross689; cross694.
For the occasional omission of ἄν in an apodosis of this kind, see cross240-242.458
The potential optative with ἄν may stand in the protasis with εἰ; as in εἰ ἔλθοιμι ἄν,
William Watson Goodwin , Syntax of the Moods and Tenses of the Greek Verb [info] [word count] [GoodwinMoodsTenses].