1. We use the indefinite article, a/an, with count nouns when the hearer/reader does not know exactly which one we are referring to:
當(dāng)聽的人或者讀者不清楚我們確切指得是哪個的時候，我們用不定冠詞a/an來形容可數(shù)名詞：

Police are searching for a 14 year-old girl.

2. We also use it to show the person or thing is one of a group:
我們也可以用來表示該人或物是同一組：

She is a pupil at London Road School.

3. We do not use an indefinite article with plural nouns and uncount nouns:
我們不用不定冠詞來修飾復(fù)數(shù)或者不可數(shù)單詞：

She was wearing blue shoes. (= plural noun)

She has short blonde hair. (= uncount noun)

4. We use a/an to say what someone is or what job they do:
我們用a/an描述他人的工作：

My brother is a doctor.

George is a student.

5. We use a/an with a singular noun to say something about all things of that kind:
我們用a/an形容一個單數(shù)名詞來表示一類事物：

A man needs friends. (= All men need friends)

A dog likes to eat meat. (= All dogs like to eat meat)