PLURAL FORMS OF NOUNS

Nouns are regularly made plural by the addition of s. A number of nouns have other plural forms.

1. es plural
a. nouns ending in sibilant sounds spelled with s, z, ch, sh, x. E.G. classes, churches, dishes, boxes but monarchs (ch spells a /k/ sound)
b. nouns ending in y preceded by a consonant: y is changed to i. E.G. ladies, countries, boundaries but toys, keys (y is preceded by a vowel)
c. one-syllable nouns ending in a single f or fe: f is changed to v. E.G. leaves, thieves, knives but sheriffs, staffs, beliefs, chiefs. Some words may also have a regular plural -- scarfs or scarves, dwarfs or dwarves.
d. nouns ending in o:
es only Negroes, heroes, echoes, potatoes, tomatoes, embargoes.
s only Terms in music - pianos, sopranos. O preceded by a vowel - cameo, radio. Others - photos, zeros.
es or s Cargoes or cargo, volcanoes or volcanos, mulattoes or mulattos

2. other types of plural
a. -en ending... children, oxen, brethren
b. internal vowel change... teeth, mice, men
c. no change... deer, sheep, series