“基”变“序”很容易，词尾加上“th”；
“一”二“三”无规律，理应单独来记忆；
“八”减“t”、“九”去“e”，“f”来把“ve”替；“二十”到“九十”，“ty”变成“tie”；
若是遇到“几十几”，只变个位就可以。
说明：
1）基数词变序数词时，通常是在词尾加上后缀-th。如：four→fourth, seven→seventh, eleven→eleventh等。
2）one, two, three的序数词分别是first, second和third,必须逐个记忆。
3）eight的序数词加h即可(eighth), nine在变ninth时应去掉e, five和twelve在加th之前要先把ve变为f, 即为fifth, twelfth。
4）twenty, thirty, forty, fifty, sixty, seventy, eighty和ninety这些整十的基数词在加th之前应先将y变为ie。
5）“几十几”的基数词在变序数词时，只要把个位数变为序数词即可，而十位数、百位数、千位数等不变。如：forty-three→forty-third, two hundred and six→two hundred and sixth等。
巩固练习：
一、单项选择
1. There are ____ days in a month.
A. thirty B. thirtieth
2. March is the ____ month of a year.
A. three B. third
3. Fifteen and ____ is thirty.
A. fifteen B. fifteenth
4. April 4th is my ____ birthday.
A. fourteen B. fourteenth
5. Lisa is ____ years old.
A. eight B. eighth
6. Today is October ____.
A. one B. first
二、把下列基数词变为序数词。
1____, 3____, 4____, 8____, 12____, 10____, 20____, 62____,
88____,99____
Key:
一、1.A 2.B 3.A 4.B 5.A 6.B
二、first, third, fourth, eighth, twelfth, tenth, twentieth, sixty-second, eighty-eighth, ninety-ninth