都具有某种能力或某种特征。

　　A horse is useful to mankind. A bird can fly.

　　3.代表所属的类别，这种用法表示人或事物的身份、性质等。

　　My father is a doctor.

　　My wish is to become a teacher when I grow up.

　　4.用在事物的“单位”前，如时间、速度、价格等有意义的名词之前，表示“每一”。

　　We often go to school twice a day.

　　Take this medicine three times a day and very soon you’ll feel better.

　　5.用力指某人某物，但不具体指任何人或任何物，只说明大体情况。

　　A boy came to see you a moment ago. I got this tool in a shop.

　　We need a car now. She is ill, she has to see a doctor.

　　6.用于某些固定词组。

　　a few 几个 a little 有点 a lot of 很多

　　have a good time 玩得高兴 have a rest 休息一下

　　She has a few friends in this city. There is a little milk in the bottle.

　　7.不定冠词的习惯用法。

　　英语中的很多习惯用法中都含有不定冠词，一般没有明确的规律可遵循，只能通过更多的积累和反复的实践才能牢固掌握。常见不定冠词的习惯短语归纳如下：

　　a moment ago 一会儿前 twice a week 每周两次

　　for a time 一段时间 in a while 一会儿后

　　in a moment 一会儿后 just a moment/minute 一会儿

　　after a time/while 一段时间之后

　　一、定冠词的用法

　　1.特指某(些)人或某(些)事物，以区别于同类中其他的人或事物。

　　The bag in the desk is mine. Is this the book you are looking for?

　　2. 指上文中已提到过的人或事物或谈话双方都知道的人或事物。

　　I bought a book from Xinhua book-shop.The book costs 15 yuan.