Count nouns have two forms: singular and plural.
The singular form refers to one person or thing:
a book; a teacher; a wish; an idea
The plural form refers to more than one person or thing:
books; teachers; wishes; ideas
Singular count nouns
Singular count nouns cannot be used alone. They must have a determiner:
the book; that English teacher; a wish; my latest idea
or a quantifier:
some new books; a few teachers; lots of good ideas
or a numeral:
two new books; three wishes
We usually add –s to make a plural noun:
book > books; school > schools; friend > friends
We add -es to nouns ending in –ss; -ch; -s; -sh; -x
class > classes; watch > watches; gas > gases; wish > wishes; box > boxes
When a noun ends in a consonant and -y we make the plural in -ies...
lady > ladies; country > countries; party > parties
…but if a noun ends in a vowel and -y we simply add -s:
boy > boys; day > days; play > plays
Some common nouns have irregular plurals:
Man > men; woman > women; child > children; foot > feet;
person > people
Plural count nouns do not have a determiner when they refer to people or things as a group:
Computers are very expensive.
Do you sell old books?