数词
表示数目多少的数词叫基数词;
表示顺序的数词叫序数词。
序数词表示排列的顺序
序数词的缩写形式为阿拉伯数字加序数词的最后两个字母.如：first---1st;second---2nd; third-3rd;
thirty-first---31st
基数词变序数词的巧记口诀
一,二,三要全变;(one-first; two-second;three- third)
其余都加th,
th里有例外,8去t,(eight-eighth) 9去e(nine-ninth)
字母f代ve(five-fifth; twelve-twelfth);
ty变成tie(twenty-twentieth)
若要变化几十几，
只变个位就可以(twenty-one-twenty-first)
数词的用法
(1)倍数表示法
a. 主语+谓语+倍数(或分数)+ as + adj. + as
I have three times as many as you.我有你三倍那么多。
b. 主语+谓语+倍数(分数)+ the size (amount，length…) of…
The earth is 49 times the size of the moon.地球是月球的49倍。
c. 主语+谓语+倍数(分数)+ 形容词(副词)比较级+ than…
The grain output is 8 percent higher this year than that of last year.
d. 还可以用by+倍数，表示增加多少倍
The production of grain has been increased by four times this year.
(2)分数表示法
构成：基数词代表分子，序数词代表分母。分子大于1时，分子的序数词用单数，分母序数词用复数：
1、/3 one-third ;3/37 three and three-sevenths
