In The Adventures of Io, a story found in Thirty-eight Latin Stories, Designed to Accompany Wheelock's Latin, the first sentence of the story is as follows:
Iuppiter, rēx deōrum, pulchram Iō amābat, sed īram Iūnōnis metuēbat.
Jupiter, king of the gods, loved beautiful Io but feared Juno's wrath.
What case is Io in this sentence, and is it a third declension feminine noun? I would have expected it to be Ionem.
A number of Greek names ending in -ō have both Latin and Greek style declensions. For example:
| Greek | Latin | |
|---|---|---|
| nom. | Dīdō | Dīdō |
| acc. | Dīdō | Dīdōnem |
| gen. | Dīdūs | Dīdōnis |
| dat. | Dīdō | Dīdōnī |
| abl. | Dīdō | Dīdōne |
One would indeed expect the typical Latin form Ionem, but Io is valid as well. The context is fortunately clear enough so that we can be confident that Io is indeed an accusative. In a more complicated sentence the role can be left quite unclear with the Greek style declension.
For unfamiliar words, it never hurts to check the dictionary. Here is what Lewis and Short have:
Īō, Iūs, and Īōn , Iōnis, f., = Ἰώ, I.a daughter of Inachus, king of Argos, beloved by Jupiter, and changed, through fear of Juno, into a cow; afterwards worshipped as an Egyptian deity, under the name of Isis. —Form Io, Ov. H. 14, 85; Prop. 2, 28 (3, 24), 17; Ov. M. 1, 588 sq.; Val. Fl. 4, 351 sq.; Hyg. Fab. 145.—Gen. Ius, Nemes. Cyn. 31.—Acc. Io, Ov. M. 1, 588; Amm. 2, 19, 29.—Abl. Io, Prop. 2, 13, 19.—Form Ion; dat. Ioni, Plaut. Aul. 3, 6, 20.—Acc. Ionem, Serv. Verg. A. 3, 153.
Annoyingly, the reference is slightly off, but here's the Ovid passage in question (Ov. Met. 1.583-585):
Inachus unus abest imoque reconditus antro
fletibus auget aquas natamque miserrimus Io
luget ut amissam.
A. S. Kline translates it thus:
Only Inachus is missing, but hidden in the deepest cave he swells his stream with tears, and in utter misery laments his lost daughter, Io.
You'll see that, like pulchram in Thirty-Eight Latin Stories, here amissam is your clue that Io is in the accusative case. The context makes that clear.
