Jonathon Owen is a copy editor and student of linguistics who "holds the paradoxical view that it's possible to be a prescriptivist and descriptivist simultaneously." Here, he looks at how people can get tripped up on words with unusual plural forms like phenomena.
Latin and Greek plurals can be tricky for English speakers. Most readers probably know that the plural of phenomenon is phenomena, but it's not uncommon to hear phenomena — and similarly inflected words like criteria — treated as singulars themselves, sometimes with regular plurals like phenomenas formed from them.
It's easy to dismiss such errors as the product of modern ignorance — people these days just don't know their Latin and Greek declensions like they used to — but these variant forms have existed almost since these words were first borrowed into English. The Oxford English Dictionary has citations for plural phenomenas dating to 1635 and singular phenomena dating to 1708, and many of these uses come from scientists and other academics who were well versed in Greek and Latin.
What gives? Are these plurals really so difficult that even classically educated scholars couldn't get it right, let alone laypeople today? Well, in a way, yes. It's not just that they're irregular. They come from a much more complicated system of inflection than English speakers are used to. Latin and Greek had elaborate systems of inflectional endings to mark not just grammatical number but gender and case.
English still marks grammatical number, usually by simply adding the plural -s ending to the stem of a word, but we no longer have grammatical gender, and case marking is limited to the personal pronouns and who, which have different forms depending on their syntactic and semantic role in the sentence: for example, I for subjects, me for objects, and my for possessives.
Ancient Greek, however, marked every word with a different ending depending on its case and number. Rather than just tacking on a plural ending, you had to know different endings for every possible combination. Phenomenon had only three separate cases, but that still gives six possible endings for every combination of case and number. The stem of the word is actually phenomen-, with -on being the ending for the nominative singular and -a the nominative plural. There were four other endings for different combinations of case and number, but we only borrowed the nominative forms.
In a nutshell, I believe this is why English speakers struggle so much with Latin and Greek plurals. We aren't used to a system that requires not just an ending to mark plurals but a different and unrelated ending to mark singulars. Pluralizing Latin and Greek words requires us to first remove one ending and then attach another, a process which we're simply not accustomed to.
To make matters worse, many of these words almost sound as if the plural has had part of the singular ending taken away instead of added on or replaced. In some pronunciations, the only difference between phenomenon and phenomena is the n at the end of the former. Many Latin plurals in English are the same; for example, stadia sounds like the -m has been deleted from stadium.
Apparently deleting something from a plural to make a singular (at least in pronunciation if not in spelling) is odd, and not just because it's different from the way English does it. The linguist Joseph Greenberg proposed forty-five language universals based on observations of thirty different languages, some of which were completely unrelated. One of them was, “There is no language in which the plural does not have some nonzero allomorphs, whereas there are languages in which the singular is expressed only by zero.” What this rather inscrutable statement means is that while there are some languages that leave the singular unmarked all the time, and there are some that mark both plural and singular at least some of the time, there are none that always leave the plural unmarked.
Stated more broadly, it's rather unusual for a plural to be formed by subtracting something from a singular, though there are occasional examples. So not only are the Greek and Latin inflectional systems highly unlike the English system for pluralization, but some of these forms are actually typologically unusual inasmuch as the singular forms appear to be derived from plurals.
So while it's easy to chalk up these mistakes to simple ignorance, I think the real reason is much more interesting. For four hundred years, English speakers have been struggling to adapt some very foreign grammar to fit our language. We've been torn between the urge to respect the original Latin and Greek grammar and the urge to regularize some odd pieces of the language that don't seem to behave the way they should.
Rather than simply decry usage errors, I find that they often tell us something interesting about the way language works, and that's a far more interesting phenomenon.