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
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。
