1、序数词前面一般要加“the”;

The first student to come to school is usually Jim.

2、序数词前面有名词所有格、形容词性物主代词时，不再使用“the”;

He is my first English teacher.

Li Lei's second book is old.

3、a /an+序数词+名词：

序数词前总可拟使用a域an, 表示的含义是“文一，再一”，常常有语言环境；

They have two children, and next year they will have a third child.

4、比赛域考试的名次前可省略“the”;

He was second in the exam.