Count nouns have two forms: singular and plural.
可数名词有两种形式：单数和复数。

The singular form refers to one person or thing:
单数形式指一个人或一件物品。

a book; a teacher; a wish; an idea

The plural form refers to more than one person or thing:
复数形式指多于一个人或一件物品：

books; teachers; wishes; ideas

Singular count nouns
单数可数名词

Singular count nouns cannot be used alone. They must have a determiner:
单数可数名词不能单独用。他们必须有一个限定词：

the book; that English teacher; a wish; my latest idea

or a quantifier:
一个量词：

some new books; a few teachers; lots of good ideas

or a numeral:
一个数词：

two new books; three wishes

Plural forms:
复数形式：

We usually add –s to make a plural noun:
我们通常在单数名词后加–s：

book > books; school > schools; friend > friends

We add -es to nouns ending in –ss; -ch; -s; -sh; -x
在以–ss; -ch; -s; -sh; -x结尾的单词后加-es：

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...
在以一个辅音跟-y结尾的单词后我们加-ies变为复数。

lady > ladies; country > countries; party > parties

…but if a noun ends in a vowel and -y we simply add -s:
但是如果一个单词以元音跟-y结尾的单词，我们直接在单词后面加-s。

boy > boys; day > days; play > plays

Some common nouns have irregular plurals:
一些普通的单词有不规则复数形式：

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:
当复数可数名词指代一类人或一类事物时，不需要限定词。

Computers are very expensive.

Do you sell old books?