Orberg Aeneis, -idis (f) dat. quomodo hoc verbum declinatur? …Accusativus? Dativus? Ablativus? ..
The genitive tells you that the stem is Aenēid- and that it belongs to the third declension, so: Aenēis, Aenēidem, Aenēidis, Aenēidī, Aenēide. (It might also be expected to follow the Greek declension with accusative Aenēida and genitive Aenēidos.)
Gratias tibi ago.