Irregular plurals, unlike regular plurals, don’t necessarily follow any pattern, and require a lot of memorization. Mastering this type of pluralization uses a different region of your brain than regular pluralization, meaning 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)

In some words, the singular form is used for both singular and plural.

• fish
• deer
• sheep
• offspring
• series
• species

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

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

Borrowed Words –i, –en, –a, –es, –ae

In words borrowed from Latin and Greek, the plural from the original language is used.

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

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