The 2nd Declension contains a mix of masculine and neuter nouns.

Masculine nouns end in –us, except puer (boy), ager (field) and vir (man). Decline these as if they were puer-us, agr-us and vir-us respectively.

Neuter nouns end in –um in the nominative singular. They decline in the same way as masculine nouns, just with the two neuter rules applied (i.e. nom. = acc.; nom. & acc. pl. end in –a.)

The vocative singular changes from –us to –e, e.g. serve! (“O slave!”), and from –ius to –i, e.g. gladi! (“O sword!”).

liberi (children), castra (camp) and arma (weapons) only have plural forms. Note that castra (literally “forts”) is translated as a singular noun.

deus (god) has the alternative forms di (nom. + voc. pl.), deum (gen. pl.) and dis (dat. + abl. pl.).