Count nouns have two forms: singular and plural.
可數(shù)名詞有兩種形式：單數(shù)和復(fù)數(shù)。

The singular form refers to one person or thing:
單數(shù)形式指一個(gè)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:
復(fù)數(shù)形式指多于一個(gè)人或一件物品：

books; teachers; wishes; ideas

Singular count nouns
單數(shù)可數(shù)名詞

Singular count nouns cannot be used alone. They must have a determiner:
單數(shù)可數(shù)名詞不能單獨(dú)用。他們必須有一個(gè)限定詞：

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

一個(gè)量詞：

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

or a numeral:
一個(gè)數(shù)詞：

two new books; three wishes

Plural forms:
復(fù)數(shù)形式：

We usually add –s to make a plural noun:
我們通常在單數(shù)名詞后加–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結(jié)尾的單詞后加-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...
在以一個(gè)輔音跟-y結(jié)尾的單詞后我們加-ies變?yōu)閺?fù)數(shù)。

lady > ladies; country > countries; party > parties

…but if a noun ends in a vowel and -y we simply add -s:
但是如果一個(gè)單詞以元音跟-y結(jié)尾的單詞，我們直接在單詞后面加-s。

boy > boys; day > days; play > plays

Some common nouns have irregular plurals:
一些普通的單詞有不規(guī)則復(fù)數(shù)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:
當(dāng)復(fù)數(shù)可數(shù)名詞指代一類人或一類事物時(shí)，不需要限定詞。

Computers are very expensive.

Do you sell old books??