Irregular Plural Nouns: Tips, Rules, and Examples

Definition of Irregular Plural Nouns

Irregular plural nouns follow some unusual rules. They don’t follow the typical rules for forming plurals, and you can’t do it just by adding an -s or—es to the singular form.

These nouns have unique plural forms. The catch is that you just have to memorize them because they don’t follow standard patterns. Here are some changes you might see:

“child” –> “children.”

“foot” –> “feet”

“mouse” –> “mice.”

Again, remember they don’t follow the normal patterns of plurals. They’re going to show up a lot in English, so keep working on them, memorizing them, internalizing them. You won’t regret it.

Common Irregular Plural Nouns: Patterns, Rules, and Exceptions

Learning the English language can be difficult. That’s why you have to familiarize yourself with the most common irregular plural nouns. They all follow unique patterns of inflection that steer away from the standard rules of adding -s or -es to form plurals.

Let’s go over the most relevant categories of irregular plural nouns:

Ending in -f or -fe

One category of irregular plurals involves nouns that end in -f or -fe. Take “wolf” and “knife” for example. For these nouns you need to change the ending to -ves, which will give you “wolves” and “knives.” But do keep in mind the exception to this rule! Take, for example, words like “roofs” and “beliefs” that are going to follow the regular plural formation pattern.

Ending in -o

Another common group of irregular plurals comprises nouns ending in -o, like “potato” and “tomato.” These nouns do follow the rule of adding -es to form plurals (“potatoes” and “tomatoes”), but there are exceptions borrowed from other languages that only require the addition of -s, such as “photos” and “pianos.”

Vowel Changes

Irregular plurals also include nouns that will go through vowel changes, like “foot” becoming “feet,” “goose” changing to “geese,” and “man” turning into “men.” These nouns remind us of who irregular the English language is, as their plural forms can change internally rather than apply the normal suffix.

Substantial Changes

Some irregular nouns feature substantial changes in their plural forms, such as “child” becoming “children,” “mouse” changing to “mice,” and “person” transforming into “people.” Why do these nouns change in such irregular ways? Well, its because they have historical roots in Old English or other languages, making them pretty unique compared to other nouns.

Unchagned Plurals

Finally, there are irregular nouns that don’t change at all in their plural forms. Some examples are “sheep,” “fish,” and “deer.” With these nouns, there is zero inflection, which makes both their singular and plural forms identical.

Category	Singular Form	Plural Form
Nouns ending in -f or -fe	wolf	wolves
	knife	knives
	life	lives
	wife	wives
	shelf	shelves
Nouns ending in -o	potato	potatoes
	tomato	tomatoes
	hero	heroes
	echo	echoes
	torpedo	torpedoes
Nouns with vowel changes	man	men
	woman	women
	foot	feet
	tooth	teeth
	goose	geese
Nouns with major changes	child	children
	person	people
	mouse	mice
	louse	lice
	ox	oxen
Nouns with zero inflection	sheep	sheep
	fish	fish
	deer	deer
	moose	moose
	aircraft	aircraft

Tricky Irregular Plural Nouns: Loanwords, Linguistic Oddities, and Morphological Challenges

Common irregular plural nouns are one thing. But mastering the trickiest and most irregular plurals is another. These nouns usually come from loanwords, linguistic oddities, and morphological exceptions that can even be confusing for native English speakers.

Latin and Greek

One category of tricky irregular plurals is nouns that have been borrowed from Latin and Greek. These loanwords will retain their original plural formations,. And that can be unfamiliar to us.

Let me give you an example: nouns ending in -us, such as “cactus” and “nucleus,” are going to follow the Latin rule of replacing -us with -i to form plurals (“cacti” and “nuclei”).

You also have nouns ending in -is, like “analysis” and “thesis,” that are going to change to -es in their plural forms (“analyses” and “theses”).

Ending in -on

Another group of tricky irregular plurals consists of nouns ending in -on, such as “phenomenon” and “criterion.” These nouns have been derived from Greek; they’re going to replace -on with -a to form plurals (“phenomena” and “criteria”).

Ending in -um

Also when you have nouns ending in -um, like “datum” and “medium,” they’re going to follow a similar pattern and change to -a in their plural forms (“data” and “media”).

Unique Transformations

Some irregular plurals undergo unique transformations that defy standard morphological rules. For instance, the noun “ox” becomes “oxen” in its plural form, while “child” changes to “children.” These linguistic oddities often have historical roots and require memorization to ensure correct usage.

Zero Inflection

Certain nouns, such as “sheep,” “fish,” and “deer,” have zero inflection which means that their singular and plural forms are going to be the same.

Its easy to get confused since they don’t change especially when you get used to adding -es to all those plurals. So watch out for this category of irregular plurals.

If you need help with all this, look at this chart that lays out these irregular forms:

Category	Singular Form	Plural Form
Latin endings (-us)	cactus	cacti
	nucleus	nuclei
	stimulus	stimuli
	syllabus	syllabi
Greek endings (-is)	analysis	analyses
	basis	bases
	crisis	crises
	hypothesis	hypotheses
Greek endings (-on)	phenomenon	phenomena
	criterion	criteria
Latin endings (-um)	datum	data
	medium	media
	stratum	strata
	curriculum	curricula
Unique transformations	ox	oxen
	child	children
	foot	feet
	goose	geese
	tooth	teeth
	mouse	mice
Zero inflection	sheep	sheep
	fish	fish
	deer	deer
	species	species
	series	series

Sources

Grammarly