Irregular Plurals: The Complete Guide

Master Tricky English Plural Forms

Why Are Some Plurals Irregular?

  • Latin/Greek origins: Words borrowed from other languages often keep their original plural forms
  • Old English: Some words retained ancient plural patterns
  • Modern trend: Many irregular plurals now accept regular "-s" forms too

🏛️ Latin-Origin Plurals

-us → -i

Singular Plural Note
cactus cacti or cactuses Both correct
focus foci or focuses Foci in science
fungus fungi or funguses Fungi preferred
nucleus nuclei Scientific term
radius radii or radiuses Radii in math
stimulus stimuli Always stimuli
syllabus syllabi or syllabuses Both common

-um → -a

Singular Plural Note
bacterium bacteria Never "bacterias"
curriculum curricula or curriculums Both accepted
datum data "Data is" now common
medium media or mediums Media (press), mediums (psychics)
memorandum memoranda or memorandums Memorandums common
millennium millennia or millenniums Both correct
stadium stadiums or stadia Stadiums preferred

-a → -ae

Singular Plural
alumna (female) alumnae
antenna antennae (insects) or antennas (TV)
formula formulae (scientific) or formulas
larva larvae
vertebra vertebrae

🏺 Greek-Origin Plurals

-is → -es

Singular Plural
analysis analyses
axis axes
basis bases
crisis crises
diagnosis diagnoses
hypothesis hypotheses
oasis oases
parenthesis parentheses
synopsis synopses
thesis theses

-on → -a

Singular Plural
criterion criteria
phenomenon phenomena
❌ Common mistake:

"The criteria is..." or "This criteria..."

✓ Correct:

"The criteria are..." or "This criterion is..."

📜 Old English & Other Irregulars

Vowel Changes

Singular Plural
foot feet
goose geese
man men
mouse mice
tooth teeth
woman women
louse lice

-en Plurals

Singular Plural
child children
ox oxen

Same Singular and Plural

Word Example
deer one deer, many deer
fish one fish, many fish (or fishes for species)
sheep one sheep, many sheep
species one species, many species
series one series, many series
aircraft one aircraft, many aircraft
salmon one salmon, many salmon

⚠️ Common Mistakes

❌ Incorrect:

"The data shows that..." (treating data as singular)

✓ Traditionally correct:

"The data show that..."

Note: "Data is" is increasingly accepted in everyday English. In academic/scientific writing, "data are" is still preferred.
❌ Incorrect:

"These phenomenons are interesting."

✓ Correct:

"These phenomena are interesting."

❌ Incorrect:

"What are your criterias for success?"

✓ Correct:

"What are your criteria for success?"

Quick Tips

💡 When in Doubt

For everyday writing, the regular "-s" plural is often acceptable for Latin/Greek words. "Cactuses" and "syllabuses" are fine in casual contexts.

✅ Professional Writing

In academic, scientific, or formal writing, use the traditional plurals (cacti, syllabi, criteria) to sound more professional.

Frequently Asked Questions

Is "octopi" correct?

Actually, "octopuses" is the most accepted form! "Octopi" is technically incorrect (it applies Latin rules to a Greek word). "Octopodes" is the Greek plural but sounds pretentious. Stick with "octopuses."

What about "alumni"?

Alumnus (male singular), alumni (male/mixed plural), alumna (female singular), alumnae (female plural). For mixed groups, use "alumni." Many people now use "alumni" for all cases or say "alums" informally.

Is "indexes" or "indices" correct?

Both! "Indexes" for books and databases, "indices" for mathematical/scientific contexts. When in doubt, "indexes" is always safe.

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