brother brethren
child children ox oxen

A few words are the same in both singular and plural; as,

sheep, trout, deer

Some nouns have two plurals which differ in meaning; as,

Singular Plural
brother brothersbrethren
pennypennies pence
peapeaspease
diediesdice

Consult a dictionary for the difference in meaning between the two plurals of each word.

Exercise 34—Compound Nouns

Singular Plural
brother-in-lawbrothers-in-law
father-in-lawfathers-in-law
court-martialcourts-martial
commander-in-chief commanders-in-chief
man-of-warmen-of-war
major generalmajor generals
goose quillgoose quills
bill of farebills of fare
spoonfulspoonfuls
cupfulcupfuls

Rule 3.—Compound nouns usually add the sign of the plural to the fundamental part of the word.