序数词是数词的一种，主要在英语语法中讲到，在汉语中表示为“第几”。此外，在生日中，描述你出生的日期时，也用到了序数词。比如：May first（5月1日）。序数词在句中可作主语、宾语、表语和定语。整十基数变序数，先把ty变成tie，要是遇上两位数，十位基数，个位序，th最后加上去。twenty-third——23rd其中lst，2nd，3rd为特殊形式，其它的都是阿拉伯数字后加th。2．当表达分数时，序数词前不用the。

相关定义

英语上数词分为两大类：基数词和序数词。

序数词是指表示顺序的数词。

记忆口诀

基数词变序数词，有规律，词尾加上-th。

一，二，三，特殊记，词尾字母t，d，d。

八去t，九除e，ve要用f替，ty把y变成i，th前面有个e。

若是碰到几十几，前用基来后用序。

整十基数变序数，先把ty变成tie，要是遇上两位数，十位基数，个位序，th最后加上去。

或者一，二，三（first、second、third）变字体；th从4上起；8加h（eighth），9减e（ninth）；用f代ve；ty变为tie。

1至100序数词：

first 1st

second 2nd

third 3rd

fourth 4th

fifth 5th

sixth 6th

seventh 7th

eighth 8th

ninth 9th

tenth 10th

eleventh 11th

twelfth 12th

thirteenth 13th

fourteenth 14th

fifteenth 15th

sixteenth 16th

seventeenth 17th

eighteenth 18th

nineteenth 19th

twentieth 20th

twenty-first 21st

twenty-second 22nd

twenty-third 23rd

twenty-fourth 24th

twenty-fifth 25th

twenty-sixth 26th

twenty-seventh 27th

twenty-eighth 28th

twenty-ninth 29th

thirtieth 30th

thirty-first 31st

thirty-second 32nd

thirty-third 33rd

thirty-fourth 34th

thirty-fifth 35th

thirty-sixth 36th

thirty-seventh 37th

thirty-eighth 38th

thirty-ninth 39th

fortieth 40th

forty-first 41st

forty-second 42nd

forty-third 43rd

forty-fourth 44th

forty-fifth 45th

forty-sixth 46th

forty-seventh 47th

forty-eighth 48th

forty-ninth 49th

fiftieth 50th

fifty-first 51st

fifty-second 52nd

fifty-third 53rd

fifty-fourth 54th

fifty-fifth 55th

fifty-sixth 56th

fifty-seventh 57th

fifty-eighth 58th

fifty-ninth 59th

sixtieth 60th

sixty-first 61st

sixty-second 62nd

sixty-third 63rd

sixty-fourth 64th

sixty-fifth 65th

sixty-sixth 66th

sixty-seventh 67th

sixty-eighth 68th

sixty-ninth 69th

seventieth 70th

seventy-first 71st

seventy-second 72nd

seventy-third 73rd

seventy-fourth 74th

seventy-fifth 75th

seventy-sixth 76th

seventy-seventh 77th

seventy-eighth 78th

seventy-ninth 79th

eightieth 80th

eighty-first 81st

eighty-second 82nd

eighty-third 83rd

eighty-fourth 84th

eighty-fifth 85th

eighty-sixth 86th

eighty-seventh 87th

eighty-eighth 88th

eighty-ninth 89th

ninetieth 90th

ninety-first 91st

ninety-second 92nd

ninety-third 93rd

ninety-fourth 94th

ninety-fifth 95th

ninety-sixth 96th

ninety-seventh 97th

ninety-eighth 98th

ninety-ninth 99th

one hundredth 100th

多位序词：

由基数词的形式变结尾部分为序数词形式来表示。

one hundred and twenty-first第一百二十一

one thousand three hundred and twentieth第一千三百二十

two hundred and fifty-second第二百五十二

相关缩写

有时，序数词可以用缩写形式来表示。主要缩写形式有：

first——1st second——2nd third——3rd fourth——4th sixth——6th twentieth——20th

twenty-third——23rd其中lst，2nd，3rd为特殊形式，其它的都是阿拉伯数字后加th。

序数词的主要形式:从第一至第十九，其中，one-first，two-second，three-third，five-fifth，eight-eighth，nine-ninth，twelve-twelfth为特殊形式，其他的序数词都是由其相对应的基数词后面添加"th"构成。

句法功能

序数词在句中可作主语、宾语、定语和表语。

The second is what I really need．第二个是我真正需要的。（作主语）

He chooses the second．他挑选了第二个。（作宾语）

We are to carry out the first plan．我们将执行第一个计划。（作定语）

She is the second in our class．在我们班她是第二名。（作表语）

注：序数词在使用时，通常前面要加定冠词the；但是如果序数词前出现不定冠词a或an时，则表示“再……”，“又……”。

We'll go over it a second time．我们再念第二遍。

We've tried it three times．Must we try it a fourth time?我们已经试过三遍了，还必须再试一次（第四次）吗？

方法举例

506房：Room 506；

第二中学：No.2 Middle School；

第五课：lesson 5；

第123页：Page 123；

7路公共汽车：Bus No.7。

不用the的情况

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。

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。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 her class.她在班上成绩排名第一。