1

指一类人或事，相当于a kind of

A plane is a machine that can fly.

2

第一次提及某人某物，非特指

A boy is waiting for you.有个男孩在等你。

3

表示“每一”相当于every，one

We study eight hours a day.

4

表示“相同”相当于the same

We are nearly of an age.

5

用于人名前，表示不认识此人或与某名人有类似性质的人或事

— Hello, could I speak to Mr. Smith?

—Sorry, wrong number. There isn't______ Mr. Smith here.

A. 不填     B. a         C. the         D. one

That boy is rather a Lei Feng.（活雷锋）

6

用于固定词组中

a couple of, a bit, once upon a time, in a hurry, have a walk, many a time

7

用于quite, rather, many, half, what, such之后

This room is rather a big one.

8

用于so(as, too, how)+形容词之后

She is as clever a girl as you can wish to meet.

9

用于抽象名词具体化的名词前

success(抽象名词)→a success(具体化) 成功的人或事

a failure 失败的人或事       a shame  带来耻辱的人或事

a pity  可惜或遗憾的事      a must 必需必备的事

a good knowledge of 精通掌握某一方面的知识