序数词作定语时，一般要与定冠词或物主代词连用。

 汤姆是他们的第二个儿子。  Tom is their second son  

 他是第一个来到这里。  He is the first one to come here  

序数词有时可与不定冠词连用，表示数量上“又—” “再一”

他又试了—次。  He tried a second time  

还要我再问他—次吗？（我已问了他两次）  Shall l ask him a third time ？

序数词的缩写形式为：阿拉伯数字加上这个词的末尾两个字母。  

表示年、月、日时，年用基数词，日用序数词。  

2005年8月15日：15th August 2005； August 15th 2005  August fifteenth two thousand and five   

基数词和序数词都可以用来给数字编号  

 The second lesson   Lesson Two （第二课）  　

 第305房间： Room three o five

 长安街76号：Chang’ an Street seventy - six

电话号码：204-2244： telephone number two o four two two four four    telephone number two o four double two double four   

十一路公共汽车： Bus eleven  the eleventh bus  

分子用基数词，分母用序数词，分子大于1时，分母加 s  

 1／3  one third  

2／5  two fifths

 数学运算的表达  

 3+5=8 Three plus five is eight    　

 Three and five is eight 

 Three plus five equals eight 

 Three added to five equals eight    

9-2=7 Nine minus two is seven    

6x5=30 Five times six is thirty

 six multiplied by five is thirty    

8÷2=4 Eight divided by two is four    

主语+谓语+倍数（或分数）+ as + 形容词 + as   

　我有你三倍那么多。  I have three times as many as you  

主语+谓语+倍数（分数）+ the size ( amount ， length … ) of …  

地球是月球的四十九倍。  The earth is forty-nine times the size of the moon  　

主语+谓语+倍数（分数）+形容词（副词）比较级+ than …  

今年比去年粮食产量增加百分之八。  The grain output is eight percent higher this year than that of last year  

还可以用 by +倍数，表示增加多少倍  

今年粮食产量增加了四倍。  The production of grain has been increased by four times this year