How Plural Nouns Are Formed

( i )     The plural of nouns is generally formed by adding -s to the singular.
For example:-     boy-boys, girl-girls, book-books, pen-pens, desk-desks, etc.
( ii )   But nouns ending in -s, -sh, -ch, or -x form the plural by adding -es to the singular.
For example:-   class-classes, kiss-kisses, dish-dishes, brush-brushes, match-matches, etc
( iii )   Most nouns ending in -o also form the plural by adding -es to the singular.
For example:-   buffalo-buffaloes, mango-mangoes, hero-heroes, potato-potatoes, etc
( iv )    A few nouns ending in -o merely add -s.
For example:-    dynamo-dynamos, solo-solos, ratio-ratios, canto-cantos, logo-logos, etc.
( v )    Nouns ending in -y, preceded by a consonant, form their plural by changing -y into -i and adding -es.
For example:-     baby-babies, lady-ladies, city-cities, army-armies, story-stories, pony-ponies, etc.
( vi )    The following nouns ending in -f or -fe form their plural by changing -f or -fe into v and adding -es.
For example:-    thief-thieves, wife-wives, wolf-wolves, life-lives, calf-calves, leaf-leaves, loaf-loaves, knife-knives, shelf-shelves, half-halves, elf-elves, self-selves, sheaf-sheaves.

• The nouns dwarf, hoof, scarf and wharf take either -s or -ves in the plural.

For example:-    dwarfs or dwarves, hoofs or hooves, scarfs or scarves, wharfs or wharves.

• Other words ending in -f or -fe add -s. 

For example:-    chief-chiefs, safe-safes, proof-proofs, gulf-gulfs, cliff-cliffs, handkerchief-handkerchiefs.

• A few nouns form their plural by changing the inside vowel of the singular.

For example:-     man-men, woman-women, foot-feet, tooth-teeth, goose-geese, mouse-mice, louse-lice.

• There are a few nouns that form their plural by adding -en to the singular.

For example:-    ox-oxen, child-children.
The plural of 'fish' is fish or fishes.In current English fish is the usual plural. Fishes can be used to refer to different kinds of fish.

• Some nouns have the singular and the plural alike.

For example:-    swine, sheep, deer, cod, trout, salmon, aircraft, spacecraft, series, species, pair, dozen, score, gross, hundred, thousand ( when used after numerals ) .
I bought three dozen oranges.
Some people reach the age of three score and ten.
The sari cost me five thousand rupees.
Stone, hundredweight.
He weighs above nine stone.

• Some nouns are used only in the plural.

1. Names of instruments which have two parts forming a kind of pair.

For example:-    bellows, scissors, tongs, pincers, spectacles.
     2.  Names of certain articles of dress.
For example:-    trousers, drawers, breeches, jeans, tights, shorts, pajamas, etc.
    3.   Certain other nouns.
For example:-    annals, thanks, proceeds, findings, environs, nuptials, obsequies, assets, chattels.

• Some nouns originally singular are now generally used in the plural.

For example:-    alms, riches, eaves.
Riches do many things.

• The following nouns look plural but are in fact singular:-

1.   Names of subjects

For example:-     mathematics, physics, electronics, etc.
2.   The word ' news '
3.    Names of some common diseases.
For example:-     measles, mumps, rickets.
4.    Names of some games.
For example:-    billiards, draughts, etc.
Mathematics is his favorite study.
No news is good news.
India won by an innings and three runs.
Measles is infectious.
Billiards is my favorite game.

• Certain Collective Nouns, though singular in form, are always used as plural.

For example:-    poultry, cattle, vermin, people, gentry.

• A Compound Noun generally forms its plural by adding -s to the principal word; as, 

Commander-in-chief      commanders-in-chief,     son-in-law   sons-in-law,    stepson--stepsons     stepdaughter     stepdaughters,    maidservant    maidservants,    passer-by     passers-by,    looker-on    lookers-on,    man-of-war      men-of-war, etc.