在英语中,使用序数词时前面要加定冠词 the,但是,在下面的情况下则不加the：
1．当序数词前有形容词性物主代词或名词所有格修饰时,序数词前不用the.
Mother was my first teacher in my life.妈妈是我生命中的第一个老师.
Tom is Lily's third boyfriend.汤姆是莉莉的第三个男朋友.
2．当表达分数时,序数词前不用the.
One fifth of the students here are from the country.这儿1/5的学生来自农村.
3．当表达“又一,再一”时 ,序数词前不用 the,但可加不定冠词.
Please give me a second chance.请再给我一次机会.
The poor woman had a third baby.那个可怜的妇女又生了一个宝宝.
4．当表达“年月日”时,尤其在朗读时,序数词前要加the；但当书写时,序数词前不用the.
He was born on June 6th,1974.他出生在1974年6月6日.
5．当序数词在句中作状语时,序数词前不用 the.
Who got there second?谁第二个到那儿的?
I finished the work first.我最先完成那项工作.
6.当表达考试（赛跑等）中获得的名次时,序数词前不用 the.
She stands first in his class.她在班上成绩排名第一.
7．当用在一些习语中,序数词前不用 the.
Business first and pleasure afterwards.先做事而后娱乐.
First come,first served.捷足先登/先到者优先.
at first sight 乍一看到
first and last 总计,一共
first of all 首先