相關定義
英語上數詞分為兩大類：基數詞和序數詞。
序數詞是指表示順序的數詞。
記憶口訣
基數詞變序數詞，有規律，詞尾加上-th。
一，二，三，特殊記，詞尾字母t，d，d。
八去t，九除e，ve要用f替，ty把y變成i，th前面有個e。
若是碰到幾十幾，前用基來後用序。
整十基數變序數，先把ty變成tie，要是遇上兩位數，十位基數，個位序，th最後加上去。
或者一，二，三（first、second、third）變字體；th從4上起；8加h（eighth），9減e（ninth）；用f代ve；ty變為tie。
1至100序數詞：
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
多位序詞：
由基數詞的形式變結尾部分為序數詞形式來表示。
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
相關縮寫
有時，序數詞可以用縮寫形式來表示。主要縮寫形式有：
first——1st second——2nd third——3rd fourth——4th sixth——6th twentieth——20th
twenty-third——23rd其中lst，2nd，3rd為特殊形式，其它的都是阿拉伯數字後加th。
序數詞的主要形式:從第一至第十九，其中，one-first，two-second，three-third，five-fifth，eight-eighth，nine-ninth，twelve-twelfth為特殊形式，其他的序數詞都是由其相對應的基數詞後面添加"th"構成。
句法功能
序數詞在句中可作主語、賓語、定語和表語。
The second is what I really need．第二個是我真正需要的。（作主語）
He chooses the second．他挑選了第二個。（作賓語）
We are to carry out the first plan．我們将執行第一個計劃。（作定語）
She is the second in our class．在我們班她是第二名。（作表語）
注：序數詞在使用時，通常前面要加定冠詞the；但是如果序數詞前出現不定冠詞a或an時，則表示“再......”，“又......”。
We'll go over it a second time．我們再念第二遍。
We've tried it three times．Must we try it a fourth time?我們已經試過三遍了，還必須再試一次（第四次）嗎？
方法舉例
506房：Room 506；
第二中學：No.2 Middle School；
第五課：lesson 5；
第123頁：Page 123；
7路公共汽車：Bus No.7。
不用the的情況
在英語中，使用序數詞時前面要加定冠詞the，但是，在下面的情況下則不加the：
1．當序數詞前有形容詞性物主代詞或名詞所有格修飾時，序數詞前不用the。
Mother was my first teacher in my life.媽媽是我生命中的第一個老師。
Tom is Lily's third boyfriend.湯姆是莉莉的第三個男朋友。
2．當表達分數時，序數詞前不用the。
One fifth of the students here are from the country.這兒1/5的學生來自農村。
Three fourths of the students here are from the country.這兒的3/4的學生來自農村。
3．當表達“又一，再一”時，序數詞前不用the，但可加不定冠詞。
Please give me a second chance.請再給我一次機會。
The poor woman had a third baby.那個可憐的婦女又生了第三個寶寶。
4．當表達“年月日”時，尤其在朗讀時，序數詞前要加the。但當書寫時，序數詞前不用the。
He was born on June 6th， 1974.他出生在1974年6月6日。
5．當序數詞在句中作狀語時，序數詞前不用the。
Who got there second?誰第二個到那兒的？
I finished the work first.我最先完成那項工作。
6. 當表達考試（賽跑等）中獲得的名次時，序數詞前不用the。
She stands first in her class.她在班上成績排名第一。
