英语翻译:fifteenth，英[ˌfɪfˈtiːnθ]美[ˌfɪfˈtiːnθ]。
第十五；十五分之一
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：
序数词前也可以使用a或an, 表示的含义是“又一，再一”，常常有语言环境；
They have two children, and next year they will have a third child.
4、比赛或考试的名次前可省略“the”;
He was second in the exam.
5、the +序数词+最高级：序数词之后有形容词和副词时，用最高级；
The long river is the second longest river in China.
He ran the third fastest in his class.
6、分数：
母序子基:即分母用序数词，分子为基数词，分子大于1，分母加－s.
三分之一 one third
三分之二 two thirds
带分数:整数+分数
two and three fourths
部分分数可以使用 half 和 quarter。
四分之一 one fourth=a quarter
四分之三 three fourths =three quarters
二分之一 one second=a half
7、日期：
5月5日
读作：May (the) fifth, the fifth of May
写作：May 5, May 5th, May the fifth, 5 May, 5th May
8、固定搭配中的序数词：
起初 at first
第一次 for the first time
首先 first of all
