English plurals: French or Latin, regular or irregular. Fortunately the vast number of plural nouns simply take the standard –s, -es, or (drop the “y” and add) –ies. However, beyond that, as usual, there is often no clear pattern.
More and more we see nouns from Latin and French adopting standard plurals. To a certain degree, this is a good thing. Aren’t regular plural forms far more accessible to the average speaker? Non-native speakers of English must rejoice to find at least a few consistent rules in English – and the less rules the better. With apologies to our French friends, although many reference books do retain the original plural forms (ex: bêtes noires), others also accept standard English rules for plurals (ex: bon mots or bons mots). Unfortunately each dictionary or reference work moves at its own individual pace – word by word – treating each noun as some kind of proverbial only child.
Tansa’s noun database contains thousands of foreign forms borrowed from French, Hebrew, Greek, Spanish and even some hybrid forms. Latin, for example, has given us the endings –i (alumni from alumnus), -ae (formulae from formula), -ices (indices from index), and –ora (corpora from corpus), to name just a few. Consider the following nouns: indexes and indices, media and mediums, data, phenomena, criteria, etc.

Antennae…
Then with yet another twist of “linguistic economy”, we sometimes elect to keep both forms, assigning each a distinct meaning. Think about antennas and antennae; Webster’s New World Dictionary, 4th edition, (among others) states that antennae (or antennas) refer to ”…the movable, jointed sense organs of most arthropods” whilst antennas could be the plural for ”…metal rods…used in receiving and sending electromagnetic waves.”

but antennas?
Interesting, but agenda really tops my list – the vagaries of “English structure”, to use the phrase rather hyperbolically. Agenda is one of the nouns that has come full circle; previously it was simply the plural of agendum. Now we normally use agenda as the singular with the plural form agendas. Poor agendum seems to have nearly disappeared from most dictionaries altogether – only a few dictionaries still even list it as a headword, and in Oxford, a mere mention, barely more than a footnote. This is example of a plural that has completely supplanted the original Latin form.
It seems we like to tinker with the most common words and tend to leave the scientific nouns alone. Thus thousands of Latin plurals yet remain in Tansa’s databases. Frequency of use is definitely the key in determining candidacy for regularization.
The scientific community illustrates a clear pedantic penchant for these Latin forms (such enthusiasm led Carl von Linné to adopt a Latin form for his own name, Carl or Carolus Linnaeus). The average person however exhibits a ready openness towards accepting regular plurals, at least for the most frequent. What do you think about criterion, criteria and criterions (yes, Webster’s 4th), and in fact one encounters bacterium, bacteria and bacterias.
Surely plural nouns in English exemplify the processes of language change quite nicely. Language is a living entity, and we all continue to sculpt it according to our fleeting whims and momentary predilections.