Advice leaflets originally produced for the Study Advice Service in the University of Hull, which holds the copyright:
Words that ended -ex or -ix in Latin had a plural in -ices (pronounced with two syllables, '-i-' as in 'in' and 'it', and 'sees'). In academic English, we can see this in such words as appendix/appendices; index/indices; helix/helices; vertex/vertices; vortex/vortices; and codex/codices. (If you do not know these words, don't worry. If you need to know them to study your own subject, you will know them. If you don't need to know them, pass on your way rejoicing.)
Words that end in -is, like crisis, emphasis and thesis form plurals in -es (pronounced with a long '-i-', as in 'machine': crises, emphases and theses (and hypotheses and syntheses). Words that ended in -um in Latin formed their plurals in -a. These include such words as curriculum, which gives the plural curricula, and an adjective curricular (don't allow your spellchecker to give you the wrong choice here!). A datum is a singular item of data. 'Datum' is a word rarely used in English though the word data, and data themselves, are essential for academic writing! (The word 'datum' is rather like 'a singular statistic'. In mapmaking and related subjects, it means 'a base line'.)
The endings -anda and -enda ~ 'the things which should be done'; corrigenda and emendenda are sometimes given at the front of a reprinting of a book : they mean 'the things that should be corrected' and 'the things that should be emended'. In Latin, the singular would be corrigendum and emendendum - but these words are rare in contemporary English, even among the most academic of academics. Many ordinary people talk of referendums; academics prefer referenda.
- An agendum is a single item of business to be transacted during a meeting all of whose business will be listed on an agenda. In Physics, people talk of a quantum of energy, or, if there are more than one of them, quanta. The plurals of maximum and minimum are maxima and minima. Scholars may refer to more than one stadium or forum as stadia and fora; not many do, these days. A singular erratum may be one item in a list of errata, or mistakes. (The ending -ata means roughly 'the things which have been done'. -atum is the singular - the thing which has been done.)
One other ending that you may come across, particularly if your subject is historical or literary, is -ana. This means roughly 'things [particularly 'writings'] to do with'. Shakespeareana is writings, etc, about Shakespeare.
The words alia (as in inter alia) and cetera (etcetera) are plurals - they mean '[among] other things', and '[and] the rest' respectively. (Note that et is the Latin for 'and', and never write "
and etc".) Omnia means 'all things', though the related omnibus is better translated as 'for everyone'. (Its short form, in modern English, is bus.)
In Greek, too, some plurals are formed in -a. (The Greek singular is usually -on.) Phenomenon is a singular. You should not write "
a phenomena" - phenomena is a plural, and should only be used when you are talking about more than one thing that has happened.
Unfortunately, there is another use, or meaning, of the ending -a. In some contexts, it shows a feminine singular. So an alumna is one female former student, where an alumnus is a male. Their plurals are alumnae and alumni respectively. (In Latin, if there are both males and females, one should use the masculine form. So a group of former students from a mixed-sex school is its alumni. They may refer to their alma mater, 'benign mother', which is another feminine singular.)
From the above, you may have deduced that the regular plural of words ending -us in Latin (masculine nouns) is -i. (Dame Edna Everage knows that the plural of gladiolus (the flower) is gladioli). Such words as radius, thus, become radii; syllabus, syllabi; and locus, loci; but other words ending in -us are neuter (neither masculine nor feminine), and end in -[vowel] + -ra, like corpus - corpora, genus - genera.
But there are many words ending -us from Latin that make their plurals regularly in English, like surplus/surpluses, campus/campuses (or, less regularly plusses). This is a particular example of the general truth that, like so much in 'good' English, it's not simple. For some words of foreign origin, it is usual (correct) to use a regular English plural -s. There is no rule to help you predict which words. You have to learn particular words and their plurals one by one.
That is one illustration of the fact that students who do not know the language concerned should never try to 'work out' the correct forms. AWE's Table of common loan words usually taking the plural form tries to give the most essential ones that students may need - or at least, have corrected by pedantic teachers - along with an idea of the system that lies behind them. It does not, of course, include every foreign plural that every student may come across.
One masculine plural from Greek is still current in academic English, and presents a problem. 'οἱ πολλοι' is used, in its English lettering as 'hoi polloi'. It means 'the many', and is used largely as a way for 'educated' people to mark their superiority to those who know no Greek. (This is sometimes fictional, of course: those who use it sometimes know no more Greek than a radish.) The phrase 'the hoi polloi' is anyway a solecism: it is equivalent to saying 'the the many', and thus has one more 'the' than necessary.
You may also want to see AWE's Table of common loan words usually taking the singular form, aqs well as the Table of common loan words usually taking the plural form.