Plural nouns are divided into regular ones and irregular, depending on their rules and characteristics when changed into the plural form.
The majority of countable nouns form their plurals by adding "es / s" as a termination, however there are a limited number of nouns that have irregular plural forms:
like names of animals, some borrowed words from other languages.
Irregular nouns form their plurals not by using the general rule of adding "s / es" termination but by changing letters in the root noun:
Man-men, woman-women, tooth-teeth, goose-geese, mouse-mice, ox-oxen, child-children.
Note: the word brother takes regular plural - brothers, but when used with the sense of a fellow member it takes an archaic irregular plural: brethren.
There are nouns that also have irregular plurals in the form of singular:
When these types of nouns are used in singular they combine with a singular verb. The company headquarters is in London. If they are used in plural they respectively require a plural verb. Three sheep are on the road.
Some nouns that form the same singular and plural in writing and general communication may be used with the terminations "s / es" when speaking about various kinds of species in science:
fishes, herrings, salmons, carps, cods, tunas, shrimps, sturgeons.
Not divisible pairs of entities are used in plural form with a plural verb; they are usually combined with the phrase "a pair of":
shorts, jeans, tights, pliers, tongs, forceps, arms, goods, troops.
There are some nouns that end in "s" but they are used in singular form with a singular verb:
news, names of diseases (measles, mumps), names of games (dominoes, billiards), and scientific subjects (mathematics, linguistics, and economics).
Some words that have the end letters –ics may take a singular or plural form depending on their meaning:
statistics is singular when speaking about science, but it is plural when it refers to a set of numbers.