The noun arma, -orum is neuter plural always according to my book. I thought neuter nouns ended in -a. What is the -orum then? Thanks.

The ending -orum is the genitive form. In the case of this word, since it doesn't have any singular forms, you give the nominative and genitive of the plural.

In the link I gave you above, scroll down to the section called Neuter Nouns ending in -um. Look at the plural column; ignore the singular for this particular word.

The noun "arma" in Latin is indeed a neuter noun in the plural form. However, it does not end in -a, as you pointed out. Instead, it ends in -orum.

In Latin, the ending -orum is called the genitive plural ending. The genitive case is used to show possession or to indicate the relationship between nouns.

To determine the form of a noun in the genitive plural, you must consider several factors, such as the noun's gender, declension, and case. In the case of "arma," it belongs to the second declension, which is characterized by masculine and neuter nouns.

Neuter nouns in the second declension form their genitive plural by replacing the -a ending of the nominative/accusative plural with -orum. Therefore, "arma" (nominative/accusative plural) becomes "armorum" in the genitive plural.

So, to sum it up, although "arma" is a neuter noun in the plural form, it does not follow the typical neuter ending -a. Instead, in the genitive plural, it takes the ending -orum, which is characteristic of second declension neuter nouns in Latin.