a+序数词表示“又一，再一”。
1、不定冠词（a/ an）+序数词表示"又一/再一"的意思。
The man had two houses in the country,and he bought a thirdone. 又买了一所房子。
2、定冠词（the）+序数词 表示"第几"的意思。
This is the second time I have come here. 第二次来。
3、the用在序数词和形容词最高级前.用来表示特定的人或事物(a/ an)用在序数词前用来表示又一/再一的意思,但不强调数的观念.只说明名词为不特定者.即不具体说明是何人何物。
如：I had two apples，but I still want a third one.我吃了两个苹果了，但我还想再吃一个（第三个）。 同学说的，“我吃了一个苹果，我还想再吃一个，这个时候用I want to eat a second one，那么有的时候我们也会说，哎，我还想再吃第二个啊”，
这时候也可以说I want to eat a second one .因为你强调“再”的含义。要是强调“这是我吃的第二个苹果”，强调“第几”，这时就用序数词，This is the second apple.
a+序数词的双语例句：
1、I ate another apple because they are so delicious. (我吃了另一个苹果，因为它们非常美味。)
2、He was still hungry after finishing his sandwich, so he grabbed another apple. (他吃完三明治后仍然感到饥饿，于是他拿了另一个苹果。)
3、She decided to have another apple for a healthy snack instead of reaching for junk food. (她决定吃另一个苹果作为健康的小吃，而不是去拿垃圾食品。)
4、The teacher told the student that he could have another apple if he finished his homework on time. (老师告诉学生，如果他按时完成作业，他可以再吃一个苹果。)