There is one more verbal mood in Koine Greek: the optative mood. Optatives are relatively rare, occurring only 68 times in the Greek NT, mostly in Luke, Acts, and Paul’s letters.
The optative mood is like a weakened subjunctive, expressing the action of the verb as a desire or wish of the speaker. On a sliding scale from probable to possible, optatives fall on the possible end. In the Koine period, the optative was being absorbed into the subjunctive mood.
Rather than have you learn all the forms of the optative, we will simply highlight the key feature and the two most common forms.
One Key Feature for Identifying Optatives
Optatives add an iota as a "mood marker" to the connecting vowel or aorist tense former. And so, for example, the present indicative λύομεν becomes λύοιμεν in the optative mood. The aorist indicative ἐλυσαμεν becomes λύσαιμεν in the optative. (Note again that aorist optatives have no augment marking past time.)
The Most Frequent Forms of Optatives
Paul often asks a rhetorical question with a firmly negative answer. For example,
Τί οὖν ἐροῦμεν; ὁ νόμος ἁμαρτία; μὴ γένοιτο!
What shall we say then? (That) the law is sin? May it never be!
—Romans 7:7
γένοιτο is the aorist middle optative, third person singular from γίνομαι (to be or become). This form occurs 17 times and usually includes the negative μὴ.
The second most common optative is εἴη—the present, active, optative, third, singular form of εἰμί. It occurs 12 times in the Greek NT.
Ἐπηρώτων δὲ αὐτὸν οἱ μαθηταὶ αὐτοῦ τίς αὕτη εἴη ἡ παραβολή.
And his disciples were asking him what this parable might be (what it meant).
—Luke 8:9