William Watson Goodwin, Syntax of the Moods and Tenses of the Greek Verb

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[*] 243. As the potential optative represents a future act as dependent on future circumstances (234), so the potential indicative originally represents a past act as dependent on past circumstances. Therefore, while ἦλθεν means he went, ἦλθεν ἄν means he would have gone (under some past circumstances). It is probable that no definite limiting circumstances were present to the mind when this form first came into use, so that ἦλθεν ἄν naturally signified merely that it was likely, possible, or probable that he went or (as we express it) that he might have gone or would have been likely to go, sometimes that he must have gone.

In this sense it appears as a past form of the potential optative, e.g. of ἔλθοι ἄν in the sense he might perchance go or he would be likely to go (in the future). The same relation appears in Latin, where credas, putes, cernas, dicas, you would be likely to believe, think, etc., are transferred to past time as crederes, putares, cerneres, diceres, you would have believed, thought, etc.¹ Here putet and putaret are precisely equivalent to οἴοιτο ἄν, he would be likely to think, and ᾤετο ἄν, he would have been likely to think.