From Textbook (Pluralization Rules) - Unit 1: The Structure of Medical Language

When a Latin singular noun ends in -a

Form the plural by replacing it with -ae

When singular form ends in -ax

Form the plural by replacing it with -aces

When a Latin singular noun ends in -us (except fetus, virus, and sinus, which are just with -es)

Form the plural by replacing it with -i

When a Latin singular noun ends in -um

Form the plural by replacing it with -a

When a Latin singular noun ends in -is

Form the plural by replacing it with -es

When a Latin singular noun ends in -ex, -ix, or -yx

Form the plural by replacing it with -ices

When a Greek singular noun ends in -is

Form the plural by replacing it with -ides

When a Greek singular noun ends in -nx

Form the plural by replacing it with -nges

When a Greek singular noun ends in -oma

Form the plural by replacing it with -omata

When a Greek singular noun ends in -on

Form the plural by replacing it with -a

When singular form ends in -ma

Form the plural by replacing it with -mata