When foreign phrases from an inflected language are used in English writing (i.e., jus divinum or koinwnia idiomatwn), is it proper to preserve the inflection of the original? Say that the phrase you copied from Greek contained a substantive in the accusative plural. Would you repeat it as accusative plural, or would you turn it back into nominative singular? Does it make any difference if it is just one word or an entire clause that you are using?
By Jove! That is a good question. As is typical with English, it depends.
In specialized writing and speaking, such as academic treatments, where the audience is expected to know English and the source, there is a large likelihood of preserving the source language inflection, especially nominative plural, posessives, and gender markers.
For words and phrases in common use, the vast majority of words come in as nominative singular. Some preserve their source language nominative plural, but the majority take regular English plural and posessive forms. Phrases, especially fixed phrases, tend to preserve their inflection in the source language.
I will try to develop a good list of examples and add them in a bit. Of course, By Jove is one of the unusual exceptions where Jupiter is found in a prepositional oblique form even though it follows an English preposition!
One good case study is Persona non grata.
Nearly tied for the lead (according to Google) for the plural are:
personas non grata
personae non grata
Distantly trailing are:
personas non gratas
persona non gratas
personae non gratae
On principle, I used to try to convince people to use “memoranda†(which this integral spell check tells me is misspelled) as opposed to “memorandums†(with which the spell check has no problem). As you might imagine, I was entirely unsuccessful.
It's not a perfect example, since it's not so much a direct import from another language accurately reflecting case, but as a nod to the original plural format, it's at least indicative of the general disregard for preserving the original grammatical format.
One final note on Persona non grata is that Google wont help to identify those who treat it as an invariant adjective.
Alma mater takes a hands down winner of a plural in Alma maters
. Losers include Almae matri, almas maters, almas mater, almae mater.
V, I think you are within your rights to defy spell checkers on that, as well as Addenda.
However, it would be a lost cause to fight Agendas or Operas.. You might see a Magnum opus from time to time, but you would be hard pressed to find an agendum.