Pluralization

English has both regular and irregular plural nouns. Regular plurals follow a rule (and other similar rules), so you generally know how to pluralize them, but irregular plurals are, well, not regular and don’t follow a “standard” rule.

Regular Plurals

Let’s start with regular plurals: regular plural nouns use established patterns to indicate that there is more than one of a thing. As was mentioned earlier, we add the plural suffix –s or –es to most words (cats, zebras, classes, foxes, heroes). Remember that when words have a foreign origin (e.g., Latin, Greek, Spanish), we just add the plural suffix –s (tacos, avocados, maestros).

When a word ends in y and there is a consonant before y, we change the y to i and add –es. Thus sky becomes skies. However, if the y follows another vowel, you simply add an –s. (donkeys, alloys). When a word ends in –f or –fe, we change the f to v and add –es (calves, leaves). However, if there are two terminal fs, or if you still pronounce the f in the plural, you simply add an –s (cliffs, chiefs).

Irregular Plurals

Irregular plurals, unlike regular plurals, don’t necessarily follow any particular pattern—instead, they follow a lot of different patterns. For this reason, irregular plurals require a lot of memorization. If you’re ever in doubt, the dictionary is there for you.

The first kind of irregular plural we’ll talk about is the no-change or base plural. In these words, the singular noun has the exact same form as the plural (sheep, fish, deer, moose). Most no-change plurals are types of animals.

The next type of irregular is the mid-word vowel change. This includes words like tooth, man, and mouse, which become teeth, men, and mice.

Note: The plural for a computer mouse (as opposed to the fuzzy animal) can either be mice or mouses. Some people prefer mouses as it creates some differentiation between the two words.

We also have the plural –en. In these words, –en is used as the plural ending instead of –s or -es.

• child → children
• ox → oxen
• brother → brethren
• sister → sistren

Note: Brethren and sistren are antiquated terms that you’re unlikely to encounter or use; however, since these are the only four words in English that use this plural, all four have been included above.

The last category of irregular plurals is borrowed words. These words are native to other languages (e.g., Latin, Greek) and have retained the pluralization rules from their original tongue.

Singular –us; Plural –i	cactus → cacti	fungus → fungi	syllabus → syllabi
Singular –a; Plural –ae	formula → formulae	vertebra → vertebrae	larva → larvae
Singular –ix, –ex; Plural –ices, –es	appendix → appendices	matrix → matrices	index → indices
Singular –on, –um; Plural –a	bacterium → bacteria	criterion → criteria	medium → media
Singular –is; Plural –es	thesis → theses	analysis → analyses	crisis → crises

The rules presented in the table above are almost always followed, but as a borrowed word becomes more popular in its usage, it can be adopted into regular pluralization. For example, formulas and appendixes are accepted words in formal situations. Also, in informal speech, cactuses and funguses are acceptable.

Note: Because of the word’s history, octopuses is preferred to octopi, but octopi is an accepted word.