English has borrowed many common words from Latin and Greek, such as amoeba, appendix, aquarium, criterion, referendum, syllabus, and virus. In their original language the plurals of such words followed that language’s rules: the Latin plural of aquarium is aquaria, the Greek plural of criterion is criteria, and so forth. However, most of these words also form regular plurals in English by adding an -s, e.g. aquariums, referendums, syllabuses.
As a rule of thumb, the Latin-style plural is appropriate to formal, scientific, or technical writing, while the English plural is better suited to everyday language. Rock guitarists use plectrums, and might be ridiculed if they called them plectra.
Choosing to use the Latin plural form when an English one is also available can smack ofpretentiousness or pomposity, as when talking of online fora rather than forums.
When deciding which to choose, it is important to bear in mind the context in which it will appear, as outlined above.
|addendum||addendums or addenda|
|aquarium||aquariums or aquaria|
|gymnasium||gymnasiums or gymnasia|
|maximum||maximums or maxima|
|memorandum||memorandums or memoranda|
|minimum||minimums or minima|
|moratorium||moratoriums or moratoria|
|referendum||referendums or referenda|
Though both curricula and curriculums are used, the first is found rather more frequently.
The word ignoramus is not a Latin noun, so its plural can only ever be ignoramuses. Both syllabuses and syllabi are used, but the first is used more often in World English, while the second is often used in American English.
The plural of octopus is another special case.
The following words always take the Latin plural:
alga => algae
desideratum => desiderata
larva => larvae
nucleus => nuclei
For a very small group of words, the choice of plural depends on the subject field: e.g. appendixes in surgery and zoology but appendices in books. In scientific work foci, formulae, indices, and vortices are regularly used, but in general writing the ordinary plural forms in -sand -es are more usual.
Three words have plurals which are regularly used as singulars in a way that many people will consider a mistake:
You should therefore avoid phrases such as these:
X a strata of society
X a strange phenomena