Irregular Plurals: Latin & Greek Origins

Level B2 · Nouns & Quantity

Latin and Greek gave English many of its academic and scientific words, and quite a few kept their original plural ending instead of simply adding -s. You'll meet these classical plurals mainly in formal, academic and scientific writing — a cactus, a criterion, a phenomenon — where the traditional form is still expected, even though everyday English is slowly regularizing a few of them.

Two endings are swapped for another, without exception on the base word:

• -us → -i (Latin): cactus → cacti, fungus → fungi, nucleus → nuclei
• syllabus → syllabi is the classical form; syllabuses also exists in everyday English, but syllabi is the one expected in academic writing
• -is → -es (Greek): crisis → crises, analysis → analyses, thesis → theses
• the spelling barely changes but the sound does — the last syllable shifts from /ɪs/ to /iːz/, so crises rhymes with 'trees', not with 'crisis'

Other endings are replaced with -a, or simply lengthened with -e:

• -on/-um → -a: phenomenon → phenomena, criterion → criteria, datum → data
• in practice 'data' is treated as a mass noun with a singular verb in everyday and much scientific English ('the data is clear'), while 'the data are' remains standard in strict academic style — both are correct, depending on register
• medium → media for communication channels (newspapers, TV, social media), but medium → mediums for people who claim to contact spirits — the same word, two different plurals, depending on meaning
• -a → -ae (mostly biology): larva → larvae, alga → algae, vertebra → vertebrae

Mistakes go in both directions. Over-regularizing adds a plain -s to a noun that already has a classical plural — cactuses, phenomenons, criterias — instead of cacti, phenomena, criteria; this is common in speech but avoided in careful academic writing. Over-correcting does the opposite: campus, bonus and virus look exactly like cactus and fungus, but they are ordinary anglicized nouns, not Latin plurals-in-i — say campuses, bonuses and viruses, never campi. A third frequent error is treating the plural itself as singular: 'a criteria' and 'this phenomena' are wrong — the singular forms are criterion and phenomenon, and the -a form is only for two or more.

You can usually tell a classical plural is needed from what surrounds the noun: numbers and quantity words above one (two criteria, several phenomena, these crises), plural verbs and pronouns (the analyses show..., the algae grow...), and set phrases like 'a set of criteria' or 'recent analyses of'. In casual speech, a few of these nouns are drifting toward regular plurals (cactuses, syllabuses, forums), but well-established scientific and academic plurals — criteria, phenomena, crises, theses, analyses — are still expected in careful writing, especially in reports, essays and research.

Practice

Forming Latin & Greek Plurals · Common Mistakes, Register & Meaning