Irregular plurals, unlike regular plurals, don’t necessarily follow any particular pattern—instead, they follow a lot of different patterns. Because of this, irregular plurals require a lot of memorization; you need to remember which nouns belong to which type of pluralization. Mastering irregulars uses a different region of your brain than regular pluralization: it’s an entirely different skill set than regular pluralization. So don’t get too frustrated if you can’t remember the correct plural. If you’re ever in doubt, the dictionary is there for you.

No Change (Base Plurals)

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. Most no-change plurals are types of animals:

• sheep
• fish
• deer
• moose

Mid-Word Vowel Change

In a few words, the mid-word vowels are changed to form the plural. This video lists all seven of these words and their plurals.

Plural –en

And last we have 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

Borrowed Words –i, –en, –a, –es, –ae

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

In informal speech, cactuses and funguses are acceptable. Octopuses is preferred to octopi, but octopi is an accepted word.

Singular -a; Plural –ae

• formula → formulae (sometimes formulas)
• vertebra → vertebrae
• larva → larvae

Singular –ix, –ex; Plural –ices, –es

• appendix → appendices (sometimes appendixes)
• index → indices

Singular –on, –um; Plural –a

• criterion → criteria
• bacterium → bacteria
• medium → media

Singular –is; Plural –es

• analysis → analyses
• crisis → crises
• thesis → theses