Health Editor’s Note: Sometimes we just have to memorize things….no way to work them out using grammatical rules. English is a difficult language to learn and you are fortunate if you leaned it as you grew up, not as a second language…Carol
100 Irregular English Plural Nouns
By Richard Nordquist ThoughtCo.
Most English nouns form their plural by adding either -s (books, bands, bells) or -es boxes, bunches, batches). These plural forms are said to follow a regular pattern.
There are no easy rules, unfortunately, for irregular plurals in English. They simply have to be learnt and remembered.
(S. Curtis and M. Manser, The Penguin Writer’s Manual, 2002)</cite
But not all nouns conform to this standard pattern. In fact, some of the most common English nouns have irregular plural forms—such as woman/women and child/children.
In addition, several nouns have alternative plurals, one regular and the other irregular.
In regard to these alternative forms, there are no strict rules to guide our use of them:
People have to learn which form to use as they meet the words for the first time, and must become aware of variations in usage. When there is a choice, the classical [irregular] plural is usually the more technical, learned, or formal, as in the case of formulas vs. formulae or curriculums vs. curricula . Sometimes, alternative plurals have even developed different senses, as in the cases of (spirit) mediums vs. (mass) media , or appendixes (in bodies or books) vs. appendices (only in books).
(David Crystal, The Cambridge Encyclopedia of the English Language, 2nd ed. Cambridge University Press, 2003