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:

　　当听的人或者读者不清楚我们确切指得是哪个的时候，我们用不定冠词a/an来形容可数名词：

　　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:

　　我们也可以用来表示该人或物是同一组：

　　She is a pupil at London Road School.

少儿英语(微博)语法入门

　　3. We do not use an indefinite article with plural nouns and uncount nouns:

　　我们不用不定冠词来修饰复数或者不可数单词：

　　She was wearing blue shoes. (= plural noun)

　　She has short blonde hair. (= uncount noun)

不定冠词a&an

　　4. We use a/an to say what someone is or what job they do:

　　我们用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:

　　我们用a/an形容一个单数名词来表示一类事物：

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

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