This occurs in the following sentence: "...ille vir audāx per āera effugere cōnstituit." I don't understand why "āera" is not in the accusative case here.
2 Answers
Āera is accusative. If you look over in the left margin, you'll see "āera acc (= āerem)". Both āera and āerem are accusative forms of āer.
Now, you might be wondering, why is āera accusative and not just āerem? It's because āer is a direct borrowing of the Ancient Greek word ἀήρ. In Greek, the accusative is ἀέρα. It's called a "third-declension Greek noun in -er". What's happening is that Roman speakers sometimes used the Greek accusative and sometimes used the native accusative. It's like the way in English we sometimes say "antennae" and sometimes "antennas".
A couple more nouns that follow this pattern are crātēr (a bowl or basin) and aethēr (the upper atmosphere, ether).
It is accusative. Nouns taken from Greek are sometimes declined in ways that replicate Greek declensions. I have copy-pasted the declension from Wiktionary. Also, Wikipedia has a good summary article: Declension of Greek nouns in Latin.
