2.基数词变序数词的方法：
(1)一般在基数词后加th
eg.four→fourth,thirteen→thirteenth
(2)不规则变化
one→first,two→second,three→third,five→fifth,eight→eighth,nine→ninth,twelve--~twelfth
(3)以y结尾的十位整数,变y为ie再加th
twenty→twentieth, forty→fortieth,ninety→ninetieth
(4)从二十一后的“几十几”直至“几百几十几”或“几千几百几十几”只将个位的基数词变为序数词。
twenty-first,two hundred and forty-fifth 口诀：
基变序，有规律;词尾要加th。
一二三，特殊记;词尾分别tdd。
八去t ，九去e ;ve则以f替。
ty 则变作 ti ; 后面还有一个e。
要是遇到几十几;只将个位变成序。
3.数词的应用： A、时间：
a. 整点：基数词 + o’clock
eg. It’s eight o’clock now.
b. 几点几分：
1.直接表达法：先小时后分
eg. 3:25 three twenty-five
2.间接表达法：先分后小时
1)(<30分钟)用past ：分钟past 小时
eg. 3:25 twenty-five (minutes) past three
2) (>30分钟)用to：(60-分钟数) to (小时数+1)
eg. 3:35 twenty-five (minutes) to four
3) 30分钟 = half 15分钟 = a quarter
eg. 3:30 three thirty = half past three
3:15 three fifteen = a quarter past three.
3:45 three forty-five = a quarter to four B、日期表达法：月日，年(或日月年)
1949年10月1日 :
October1st , nineteen forty-nine
=the first of October, nineteen forty-nine
2000年: the year two thousand
=twenty hundred
2001年: twenty o one
3月1日: March the first = the first of March C、表编号：
第207房间：Room 207
第五课： Lesson 5 = the fifth lesson D、序数词与不定冠词(a; an)连用表“又一;再一”
eg. You’ve done it three times. Why not try a fourth time?
E、分数的表达：分子(基数)、分母(序数)
eg. one third 三分之一 two thirds 三分之二 注意：
1.分子超过1时，分母加s
2.含分数的短语作主语由分数后的词决定谓语
eg.1)One third of the students are girls.
2)One third of the milk is mine.
3.分数的特殊形式
1)one third = a third
2)one fourth = a quarter
three fourths = three quarters
3)one second = a half F、表约数：
表不确定的数字时，数词用复数。
若带名词，再加of
如：hundreds of(数百，成百上千的)，thousands of(数干，成千上万的)，millions of(数百万的) G、数学运算的表达
eg.3+5=8 Three plus/and five is eight.
9-2=7 Nine minus two is seven.
6x5=30 Five times six is thirty
8÷2=4 Eight divided by two is four.