1. 用于序数词前：用在序数词前表示“再一”“又一”。如：

She thanked him a second time.

她再次向他道谢。

Shall I ask her a third time?

我还要问她一次吗?

2. 用于由动词转化来的名词前：表示一次、一番等义(通常与 have, take, make, give 等动词连用)。如：

We’ve already had a try at it.

我们已经试过一下。

I went down to the beach and took a swim.

我到海边去游了游泳。

3. 用于专有名词前：相当于a certain，意为“一个”“某个”。如：

The house next door has been bought by a Mr Jones.

隔壁的房子被一个叫琼斯的先生的人买下了。

4. 用于抽象名词前：使之具体化，表示与该之相关的具体的人或事。如：

He was a success in business.

他事业成功。

It’s a pleasure to talk with you.

同你谈话是件令人愉快的事。