Advice leaflets originally produced for the Study Advice Service in the University of Hull, which holds the copyright:
English grammar is very simple, by comparison with that of many other languages. For one thing, nearly all nouns (= names of things) form their plurals by adding -s (or -es) - the regular plural form. There are some irregular plurals in native English - for example, child → children, goose → geese, woman → women, sheep → sheep; but the vast majority of nouns form their plurals regularly.
In other languages, there are several different ways of forming plurals. Each group of nouns in a given language whose singular ends in one way tends to form its plurals in a particular, regular pattern. But another group, with a different singular ending, will normally form its plural in a different regular pattern.
In academic English, many words come from such languages. Most are from Latin and Greek, the ancient languages of Rome and Greece which were for centuries the languages of University teaching. Many of these words form plurals in ways other than using -s. These plurals may be perfectly regular in the original languages, but they look irregular in English. If you want to use academic vocabulary well, you should use the correct plurals of these words. Sometimes these are the forms from the original languages; sometimes they are not. Gosh, academic English is difficult sometimes!
In the beginnings of modern science, much use was made of Latin and Greek to supply words for new discoveries. In biology, all students should know the Linnaean system of naming each group of living things by reference to its species and genus, etc. The names are nearly always in a Latin form. Higher levels of the Linnaean system are often in the forms of plurals: mammalia is the Latin for 'mammals', flora for '(the goddess of) flowers', and arthropoda for 'arthropods' (spiders, etc). Words from the Linnaean system are not covered further in this leaflet: books on Biology, and particularly taxonomy, should supply you with all that you need.
In astronomy, many stars are known by Latin (or, more rarely, Greek) names. These sometimes have modified endings: the constellation Centaurus (the centaur) has a star called proxima centauri. This means 'the nearest star of the centaur' - the ending -i here shows a possessive. In meteorology, some kinds of cloud are known by Latin names ending in -us (e.g. cumulus) in the singular, and -i in the plural (cumuli). The clusters of stars that look like clouds are nebulae in the plural; one is a nebula.
In medical subjects, topics felt to be taboo were often "buried in the decent obscurity of a learned language" (Oscar Wilde). If you are studying in a medical field, you may need to use such terms (e.g. excreta and genitalia), but if you are, you should be taught them as your studies proceed.
Law, too, is a subject whose vocabulary owes a great deal to Latin. Both Anglo-Saxon and European ("Roman") systems derive material from the lawgivers of the Roman Empire. So there are many words and phrases, like ius (plural: iures), lex (leges) and ultima ratio regis (ultima rationes regis), whose pluralisation you should learn through your course. (It is usual to mark words and phrases that are clearly 'foreign' by printing them in italic. Words that have been absorbed into English, however, do not have italics - no matter how un-English their plurals are.)
History and archaeology often examine aspects of Roman and Greek culture. So they use appropriate vocabulary - often with great care for grammatical accuracy. Remember, too, that until the Protestant reforms of the 16th Century, Latin was the language of all Christianity, and thus of most literacy and education. In the "Roman" Catholic church, it was used until the 1960s. So religious subjects also tend to use Latin with more care than most others.
You may want to see Some examples of foreign plurals used in academic English, Table of common loan words usually taking the singular form or Table of common loan words usually taking the plural form.