Accent behavior and plural formation for nouns with the diphthongs οι and αι; examples of accent shifts in singular and plural (e.g., ἄνθρωπος→ἄνθρωποι, Μήδεια→Μήδειαι), and alternations from -ος to -αι/-οι.
Αἱ δὲ εἰς 'οι' τῇ ἑνικῇ ἀκολουθοῦσι εὐθείᾳ καὶ αὐτῆς ἔχουσι τὸν τόνον, ἄνθρωπος ἄνθρωποι, πλησίος πλησίοι, ἱστός ἱστοί. ἔνθεν ἀπὸ μὲν τοῦ ὁ χρυσός οἱ χρυσοί ὀξύνεται, ἀπὸ δὲ τοῦ ὁ χρυσοῦς οἱ χρουσοῖ περισπᾶται, καὶ ἐπὶ τῶν ὁμοίων ὁμοίως. Αἱ δὲ εἰς 'αι' μονογενεῖς μὲν οὖσαι ἐπὶ τῆς αὐτῆς συλλαβῆς ἔχουσι τὸν τόνον τῇ εὐθείᾳ, Μήδεια Μήδειαι, μοῦσα μοῦσαι, ἄελλα ἄελλαι, τοξότης τοξόται, κοχλίας κοχλίαι, Αἰνείας Αἰνεῖαι, Ἀτρείδης Ἀτρεῖδαι, εἰ καὶ μὴ τὸν αὐτὸν διὰ τὸν κανόνα. ἡ γὰρ 'αι' ἀντὶ κοινῆς λαμβάνεται. οὐδὲ ὄντως οὖν Ἀττικοὶ τὸ αἴτιαι, ἥμεραι, εὐπράξιαι, τιμώριαι προπαροξύνουσι οὐδὲ τὸ τραγῴδιαι, κωμῴδιαι. Αἱ εἰς τὴν 'αι' δίφθογγον λήγουσαι εὐθεῖαι τῶν πληθυντικῶν παρεσχηματισμέναι ἀρσενικοῖς, ἐὰν μὲν ὦσιν ἀπὸ τῶν εἰς 'ος', τῷ τόνῳ τοῦ ἀρσενικοῦ ἀκολουθοῦσι, τουτέστι τῆς εὐθείας τῶν πληθυντικῶν τοῦ ἰδίου ἀρσενικοῦ, οἷον ποικίλοι ποικίλαι, ἐναντίοι ἐναντίαι, σοφοί σοφαί, καλοί καλαί, λίθινοι λίθιναι, δρύϊνοι δρύϊναι, ἅγιοι ἅγιαι, τίμιοι τίμιαι, δίκαιοι δίκαιαι, ὅσιοι ὅσιαι, Λύκιοι Λύκιαι τὸ τριγενές. Λυκίαι δὲ τὸ μονογενὲς πρὸς τὸ Λυκία· ἥμεροι ἥμεραι, ἡμέραι δὲ
The forms in -οι in the singular follow the nominative and have their accent from it: ἄνθρωπος ἄνθρωποι, πλησίος πλησίοι, ἱστός ἱστοί. Hence from ὁ χρυσός comes οἱ χρυσοί with an acute, but from ὁ χρυσοῦς comes οἱ χρουσοῖ with a circumflex, and similarly in the like cases. The forms in -αι, being of one gender, have the accent on the same syllable as the nominative: Μήδεια Μήδειαι, μοῦσα μοῦσαι, ἄελλα ἄελλαι, τοξότης τοξόται, κοχλίας κοχλίαι, Αἰνείας Αἰνεῖαι, Ἀτρείδης Ἀτρεῖδαι, though not the same accent in accordance with the rule; for -αι is taken as equivalent to a common ending. Accordingly the Attics do not in fact accent αἴτιαι, ἥμεραι, εὐπράξιαι, τιμώριαι with a proparoxytone, nor τραγῴδιαι, κωμῴδιαι. The nominatives plural ending in the diphthong -αι, formed after masculine patterns, if they are derived from forms in -ος, follow the accent of the masculine—that is, of the nominative plural of the corresponding masculine—e.g. ποικίλοι ποικίλαι, ἐναντίοι ἐναντίαι, σοφοί σοφαί, καλοί καλαί, λίθινοι λίθιναι, δρύϊνοι δρύϊναι, ἅγιοι ἅγιαι, τίμιοι τίμιαι, δίκαιοι δίκαιαι, ὅσιοι ὅσιαι, Λύκιοι Λύκιαι in the three-gender form. But Λυκίαι is the one-gender form corresponding to Λυκία; ἥμεροι ἥμεραι, and ἡμέραι …