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英语序数词前面不一定都要加the，应视情况而定。当序数词单纯地表示“第几”的时候就加the，当序数词用于“又一”的意思时则用a，当序数词前有人称代词时，不加the。
序数词什么情况下不加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.
Three fourths of the students here are from the country.这儿的3/4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第三个宝宝.
4．当表达“年月日”时,尤其在朗读时,序数词前要加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 her class.她在班上成绩排名第一.