Modern English is a combination of many languages and has become an international language. The problem is that the combination is not always standardised. That makes learning English more difficult.
Many nouns which have irregular plurals are words taken from other languages. In modern English we add -s or -es to the end of a word to create the standard plural. When words are borrowed, English often uses the original spelling and plural form.
English is in the Indo-European language family. All the Indo-European languages are inflected, meaning that the spelling of a word is changed depending on how it is used. For the most part English has been simplified by history and has discarded most inflections. The part that remains is the plurals. The Chinese languages are not inflected. So Chinese words used in English often do not have a plural form.
(Most examples are from the British National Corpus.)
The first example is the Chinese currency, the yuan. Unlike dollars, it has no inflected plural form.
He threatened to cut him off without a yuan if he did.
In science, Latin and Greek are the languages most often used as a source of new words. Many, but not all, English words ending in -us have an -i plural. The words fungus/fungi are Latin words for mushrooms and related organisms.
Morels, simply put, are the Most Delicious Edible Fungus.
In May/June butterflies can be seen along the open rides and in autumn there is a rich variety of fungi.
Other words from Latin and Greek have unusual plurals. From Greek (by way of Latin) we find phenomena/phenomenon. The word phenomenon is the singular. The original plural form phenomena is still used, but is slowly being replaced by the less common phenomenons. In the first example phenomenon is singular.
Crime is a complex social phenomenon with no single cause or solution.
(Hansard extracts 1991–1992)
In this second sentence is the plural word phenomena.
The objects and phenomena that a physics book describes are simpler than a single cell in the body of its author.
(The Blind Watchmaker)
The Latin word data is plural and the singular datum is usually ignored. (A quick corpus analysis show that datum is used slightly more often in UK English compared to American English.)
Fowler had similar basic data but presented in a quite different way.
The language that has had the greatest influence on English is French. The French-speaking Normans invaded England in 1066 at the Battle of Hastings and ruled until the Battle of Bosworth Field in 1485. Most French words in English have been changed to use the standard English plurals. The French word bureau still sometimes uses the original French plural, bureaux. I recommend sticking to the more standard form, bureaus.
The Legal Policy Division advises government departments and bureaus on whether proposed legislation, or a particular policy, is consistent with the Basic Law,...
(A HK government website)
The Government Secretariat was re-organised into 12 bureaux with effect from 1 July 2007.
Some borrowed words follow the spelling and grammar rules of the original language. This is where we get the additional language patterns for plurals. Here is a chart of some of the more common foreign plurals. Chinese words in English, such as yuan or dim sum, usually have no plural form.
-i or -es
-s and rarely -a
onion/onions, balloon/balloons, apron/aprons, polyhedron/polyhedrons or polyhedra
-s and rarely -a
eaux or eaus
Next week I'll explain -f or -fe endings:
-f or -fe
-ves or -s
loaf/loaves, belief/beliefs, knife/knives
(Warning!) The chart above is a guideline, not a rule; when in doubt check a dictionary.
A good easy suggestion is to use the -s or -es endings if you are not sure and don't have time to look the word up in a dictionary. Usually you will be right. When two forms are used, the standard -s or -es ending is the better choice. The plural plateaus is a better, more standard, choice than plateaux. (British English uses both plurals, even though American English tends only to use plateaus.) The same is true of using polyhedrons rather than the less common polyhedra. Writing datas is wrong, but it is a very easily understood mistake. (The Star Trek character, Commander Data, should be Commander Datum to be grammatically correct.) Slowly the language will change and become more standardised.