Second declension nouns are usually masculine, but they can be neuter. Nouns that fall into this category are: mercatum (market) and bellum (war). These are the endings for neuter nouns ending in -um:

N : –um

G : –ī

D : –ō

Acc : –um

Abl : –ō

The endings for the Latin word for spear:

N : pīlum

G : pīlī

D : pīlō

Acc : pīlum

Abl : pīlō

The endings for the Latin word for fodder:

N : pabulum

G : pabulī

D : pabulō

Acc : pabulum

Abl : pabulō