Nouns with two plural forms

Certain nouns have two plural forms. An example is the noun fish. Both fish and fishes can be used as the plural of fish. However, in modern English it is more common to use fish as the plural form. The form fishes is sometimes used to talk about different kinds of fish.

This pond abounds in fish.
Fishes of all kinds are sold in this market.

Nouns with identical singular and plural forms

Some nouns have identical singular and plural forms. Examples are: sheep, swine, aircraft, spacecraft, series, species, pair etc. Note that the nouns pair, dozen, hundred, thousand etc., do have the plural forms pairs, dozens, hundreds and thousands. But after a numeral we write the singular form.

I bought three dozen mangoes. (NOT I bought three dozens mangoes.)
I bought this camera for two hundred dollars. (NOT …for two hundreds dollars.)

Compare:

She has dozens of handbags.
She has two dozen handbags.

Some nouns look plural, but are in fact singular. Examples are: mathematics, electronics, physics, billiards etc. The names of some common diseases are also singular. Examples are: measles, rickets, mumps etc.

Physics is his favorite subject.
Measles is infectious.

Means can be singular or plural. But when means is used with the meaning of ‘wealth’ it is considered plural.

His means are small, but he has incurred no doubt.

Some nouns have two forms for the plural, each with a different meaning.

Examples are:

Brother (singular), brothers, brethren (plural)
Here ‘brothers’ means sons of the same parent. ‘Brethren’ means members of a society or a community.
Similarly cloth has two plural forms – cloths and clothes. Here ‘cloths’ means pieces of cloth. ‘Clothes’ means garments.
Abstract nouns

Abstract nouns have no plural forms. They are uncountable. Examples are: hope, beauty, bravery, charity, kindness, information etc.
Note that when these nouns do appear in the plural, they are treated as countable nouns.

Examples are: provocations (= Instances of provocation)
Kindnesses (= Acts of kindness)

Material nouns are also uncountables and are not used in the plural. Examples are: copper, iron, gold, tin, wood etc. When these nouns are used in the plural they become countables with changed meanings. Examples are:

Coppers = copper coins
Irons = fetters
Woods = forests