The Noun : Number

Consider the following pair of words.
Tree, trees.
Box, boxes.
Ox, oxen.
Man, men.

The first word of each pair denotes one thing, the second word of each pair denotes more than one.

A Noun that denotes one person or thing is said to be in the Singular Number as boy, girl, cow, tree, book, etc.
A Noun that denotes more than one person or thing is said to be in the Plural Number as boys, girls, cows, trees, books.

There are two Numbers in English - Singular and the Plural.

How Plurals are formed

1. The Plural of Nouns is generally formed by adding -s to the singular as,
boy, boys
girl, girls
pen, pens.

2. But Nouns ending in -s, -sh, -ch or -x form the plural by adding -es to the singular,
class, classes
kiss, kisses
tax, taxes
brush, brushes
box, boxes,
watch, watches.

3. Most nouns ending in -o also form the plural by adding -es to the singular as,
buffalo, buffaloes
mango, mangoes
potato, potatoes
hero, heroes
echo, echoes
volcano, volcanoes
cargo, cargoes.

4. A few nouns ending in -o, generally those which are in less common use merely add -s, as,
dynamo, dynamos
solo, solos
ratio, ratios
piano, pianos
photo, photos
stereo, stereos.

5. Nouns ending in -y preceded by a consonant, form their plural by changing -y into -i adding -es as,
baby, babies
lady, ladies
city, cities
pony, ponies
army, armies
story, stories.

6. Several nouns ending in -f or -fe form their plural by changing -f or -fe into v and adding -es as,
thief, thieves
wife, wives
wolf, wolves
life, lives
knife, knives
calf, calves
shelf, shelves
loaf, loaves.

Exceptions:-
chief, chiefs
dwarf, dwarfs
hoof, hoofs
gulf, gulfs
brief, briefs
belief, beliefs
grief, griefs
roof, roofs
proof, proofs.

A few nouns form their plural by changing the inside vowel of the singular as
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 as,
Ox, oxen, child, children.

Some nouns have the singular and the plural alike as,
swine
sheep
deer
cod
trout
salmon
dozen
score
gross
hundred.

Example:-
I bought three dozen oranges.
Some people reach the age of three score and ten.

Some nouns are used only in the plural.
1. Names of instruments which have two parts forming a kind of pair as
Bellows, scissors, tongs, pincers, spectacles.
2. Names of certain articles of dress, as
Trousers, drawers, breeches.
3. Names of diseases as
Measles, mumps.
4. Names of games as
Billiards, draughts.

Some nouns originally singular are now generally used in the plural as
Alms, riches, eaves.

The following plural forms can be used in singular as,
Mathematics, physics, mechanics, politics, news, innings.
Mathematics is his favorite study.

Certain Collective Nouns though singular in form, are always used as plurals, as
Poultry
cattle
people
gentry.

Example:-
These poultry are mine.
Whose are these cattle?
There are few gentry in this town.

Note- As a Common Noun, 'people' means a nation and is used in both singular and plural as
1. The French are a hard-working and brave people.
2. There are many different peoples in Europe.

A Compound Noun generally forms its plural by adding -s to the principal word as,
Singular - Plural
Son-in-law - sons-in-law
Man-of-war - men-of-war
Step-son - step-sons
Maid-servant - maid-servants

Some nouns have different meaning in the singular and the plural.
Singular - Plural
Advice: counsel - Advices: information
Good: benefit - Goods: merchandise
Respect: regard - Respects: compliments
Force: strength - Forces: troops

Letters, figures and other symbols are made plural by adding an apostrophe and s as,
There are more e's than a's in this page.
Add two 5's and three 2's.

Abstract Nouns have no plural.
Hope, charity, love, kindness.

Names of substances or materials known as Material Nouns, are also not used in the plural.
Copper, Tin, Iron, wood.