When words deviate from standard spelling rules, they are considered "irregular."

An irregular noun is a noun that behaves in a different way than most nouns in the language. In English, irregular nouns are those that take a different plural form than regular nouns.

Typically in English you form plurals of nouns by adding the suffix -s or -es to the end of the singular form of the word. For instance, the plural of “cake” is “cakes,” the plural of “noun” is “nouns,” and the plural of “bench” is “benches”.

An irregular noun doesn't conform to these rule. You don’t say “womans” to refer to a group of women or “sheeps” to refer to a herd of sheep, unless you’re either a second-year toddler or a first-year ESL student.

The plurals of irregular nouns have to be memorized, but the good news is that there are some rules.

Nouns Ending in -f or -fe

For those nouns that end in -f or -fe, change the -f to -v and add the suffix -s or -es to form plural. For instance, the plural of wolf is wolves and the plural of leaf is leaves. There are some exceptions here: For instance, the plural of chef is chefs, not cheves.

Nouns Ending in -o

For nouns ending in -o, form a plural by adding the suffix -es. For instance, tomato becomes tomatoes, and hero becomes heroes.

Irregular Plurals of Latin and Greek Words

Nouns of Greek or Latin origin typical have irregular plurals. For nouns ending in -us replace it with -i to form plural (e.g. cactus - cacti, nucleus - nuclei).

For nouns that end in -on or -um, replaces these endings with -a to form plural (bacterium - bacteria, phenomenon - phenomena, curriculum - curricula).

For nouns ending in -is, replace this ending with -es to form plural (analysis - analyses)

For nouns ending in -ix and -ex, replace these endings with the ending -ices to form plural. For instance, index-indices, appendix-appendices.

Irregular Noun That Changes Completely

Some nouns take a completely different form when forming a plural. Often this involves a vowel change, as in mouse-mice, foot-feet, woman-women. Sometimes the change is more substantial, as in child-children, ox-oxen.

The plural forms of these nouns have formed historically and simply need to be memorized.

Irregular Nouns That Don't Change When Plural

Similarly, some irregular nouns don't change at all when in plural form. For instance, the plural of sheep is sheep, the plural of bison is bison, and the plural of deer is deer.