Learn English Basic Grammar With Simple Examples

Notice the change of form in the second word of each pair:
{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, bird, tree, book, pen.
A Noun that denotes more than one person or thing, is said to be in the Plural Number;as,
Boys, girls, cows, birds, trees, books, pens.
Since there are two numbers in English--The Singular and the Plural.
The plural of noun is generally formed by adding-s to the singular;as,
{Boy--Boys } {Girl--Girls} {Pen--Pens} {Desk--Desks}