English has many nouns that do not follow the regular plural pattern. Some nouns change internally (man → men, woman → women, child → children, tooth → teeth, foot → feet, mouse → mice). Other nouns have the same singular and plural form (sheep, fish, deer). Still others come from other languages and keep their original plural forms (datum → data, crisis → crises, phenomenon → phenomena). Learning the most common irregular plurals is essential for accurate speech and writing.
You know that English typically forms plurals by adding -s or -es (cat → cats, bus → buses). That's the default rule, applied automatically to new words — when a new tech product launches, we say "two iPhones," not some invented plural. But English has been spoken and written for over a thousand years, absorbing vocabulary from Old English, Latin, Greek, French, and other languages. Many of those borrowed words arrived with their own pluralization systems and, for various reasons, kept them. Irregular plurals are the historical residue of those other systems.
The internal-change plurals — man → men, woman → women, foot → feet, tooth → teeth, goose → geese, mouse → mice — come from Old English and represent a process called vowel mutation or umlaut, where the plural was once signaled by changing the vowel sound rather than adding a suffix. These are among the oldest, most frequently used words in the language, which is exactly why they survived regularization. High-frequency words resist change because speakers encounter them so often that the irregular form becomes second nature. New, rare words almost always get the -s treatment; old, common words often keep their historical forms.
Zero-plural words — sheep, deer, fish, species, series — typically refer to animals historically hunted or kept in herds, or to abstract categories. English historically treated these as collective or mass nouns: "I caught three fish" works like "I drank three cups of water," treating the referent as a substance rather than a set of individuals. This pattern is most consistent for game animals and formal taxonomic categories, which is why "two species" and "four deer" feel natural while "two cats" does not.
Latin and Greek plurals entered English through scholarly and scientific writing, which is why they cluster in academic vocabulary: datum → data, analysis → analyses, criterion → criteria, phenomenon → phenomena, syllabus → syllabi, thesis → theses. The educated communities that first used these words preserved the original classical forms as a mark of precision and learning. Today the picture is mixed: "forums" is replacing "fora," "indexes" competes with "indices," and "agendas" has almost entirely displaced "agendae." A good rule of thumb is that the more technical or formal the context, the more likely the classical plural is expected — "data" in a scientific paper, "datas" never. When in doubt, the classical plural signals familiarity with the academic register in which these words circulate.