English is often considered one of the most flexible languages in the world. One reason for this flexibility is that English has borrowed thousands of words from other languages over centuries. As these words entered English, many brought along their original grammatical rules, including how they form plurals.
This is why we encounter words such as:
To many learners, these plural forms appear inconsistent and confusing. Why do some words have two accepted plural forms? Why do some preserve their foreign endings while others adopt standard English plurals?
The answer lies in the history of language, etymology, academic traditions, and the natural evolution of English itself.
Most English nouns form plurals by adding:
Examples:
However, English contains many words borrowed from:
Many of these words retained their original plural endings.
As a result, English developed multiple pluralization systems operating simultaneously.
Latin has contributed thousands of words to English, especially in:
Many Latin nouns preserve their traditional plural forms.
Latin nouns ending in -us often change to -i.
Examples:
| Singular | Traditional Plural |
|---|---|
| Cactus | Cacti |
| Fungus | Fungi |
| Nucleus | Nuclei |
| Stimulus | Stimuli |
| Radius | Radii |
Modern English also accepts:
Although specialists often prefer the classical forms.
The word "cactus" comes from Latin.
In Latin grammar:
When English adopted the word, it also adopted its Latin plural.
Today both are acceptable:
Example:
✓ The greenhouse contains many cacti.
✓ The greenhouse contains many cactuses.
Both are grammatically correct.
Latin nouns ending in -a frequently form plurals with -ae.
Examples:
| Singular | Classical Plural |
| Formula | Formulae |
| Larva | Larvae |
| Vertebra | Vertebrae |
| Alga | Algae |
However, modern English increasingly uses:
especially outside academic writing.
This is one of the most famous examples of multiple plurals.
Usually found in:
Example:
"The formulae used in the study were derived from Newtonian mechanics."
Common in:
Example:
"The spreadsheet contains several formulas."
Both are correct, but context often determines preference.
The word "index" comes from Latin.
It has two accepted plurals:
Common in:
Example:
"The book contains several indexes."
Preferred in:
Example:
"The stock market indices rose sharply."
This distinction developed naturally as different professional fields adopted different conventions.
Latin nouns ending in -um often become -a.
Examples:
| Singular | Plural |
| Curriculum | Curricula |
| Datum | Data |
| Medium | Media |
| Bacterium | Bacteria |
| Memorandum | Memoranda |
Modern English sometimes uses:
especially in business contexts.
Greek has also contributed many words to English.
These words often retain Greek plural endings.
Examples:
| Singular | Plural |
| Criterion | Criteria |
| Phenomenon | Phenomena |
| Automaton | Automata |
Example:
Incorrect: "Many criterion exist."
Correct: "Many criteria exist."
Greek nouns ending in -is typically become -es.
Examples:
| Singular | Plural |
| Analysis | Analyses |
| Thesis | Theses |
| Crisis | Crises |
| Diagnosis | Diagnoses |
| Hypothesis | Hypotheses |
These forms are still strongly preserved in modern English.
Several factors explain dual plural forms.
English gradually simplifies foreign grammar.
Example:
People naturally prefer familiar patterns.
Different professions maintain different standards.
Examples:
Both forms remain valid.
British English often preserves classical plurals longer.
Examples:
American English tends to favor:
Universities, legal institutions, and scientific organizations have historically maintained Latin and Greek grammar.
This preservation has helped classical plurals survive for centuries.
Not all unusual plurals come from Latin or Greek.
English also inherited native irregular plurals.
Examples:
| Singular | Plural |
| Child | Children |
| Foot | Feet |
| Tooth | Teeth |
| Mouse | Mice |
| Goose | Geese |
| Person | People |
These forms originate from Old English grammatical patterns.
Linguists have observed a gradual movement toward English-style plurals.
Examples:
| Traditional | Modern Alternative |
| Cacti | Cactuses |
| Formulae | Formulas |
| Memoranda | Memorandums |
| Appendices | Appendixes |
| Syllabi | Syllabuses |
Neither form is inherently wrong.
Usage depends on:
| Singular | Plural 1 | Plural 2 |
| Cactus | Cacti | Cactuses |
| Formula | Formulae | Formulas |
| Index | Indices | Indexes |
| Appendix | Appendices | Appendixes |
| Memorandum | Memoranda | Memorandums |
| Syllabus | Syllabi | Syllabuses |
| Octopus | Octopuses | Octopi* |
| Curriculum | Curricula | Curriculums |
*Interestingly, "octopi" is technically based on a mistaken assumption that octopus is Latin. The preferred modern plural is "octopuses."
Use classical plurals when:
Use English plurals when:
Always prioritize consistency within the same document.
Many people believe:
Octopus → Octopi
However, the word "octopus" comes from Greek, not Latin.
Possible plurals include:
This demonstrates how language often evolves according to common usage rather than strict historical rules.
The existence of multiple plurals in English reflects the language's rich multicultural history. Words such as "cacti," "formulae," and "indices" preserve traces of Latin and Greek grammar, while their English alternatives show how the language continuously adapts and simplifies itself.
Rather than being mistakes, multiple plural forms represent different stages of linguistic evolution. Understanding their origins helps writers choose the most appropriate form for their audience while appreciating the fascinating history hidden within everyday words.
English is not merely a language—it is a living museum of centuries of cultural exchange, and its plural forms are among its most interesting exhibits.
