数词分为两种：基数词(表示数目)和序数词(表示顺序)。
1. 基数词的基本用法用作基数词单位的hundred, thousand, million, billion 通常不带复数词尾-s，但若用于表示数百、数千、数百万这样的泛指，则用复数。
About two thousand people died in the earthquake.
Thousands of people go to the seaside every year.
2. 表示年龄和年代的用法表示整十的基数词用复数形式可以表示人的大概岁数或某个年代。
He is in his early twenties. 他的年纪在二十出头。
This story took place in the 1930s.
1.序数词的构成大多数的序数词是由基数词加-th构成，如：seven—seventh, ten—tenth
以-t结尾的基数词只在词尾加-h，如：eight—eighth
以-ve结尾的基数词改-ve为f再加-th，如：five—fifth, twelve—twelfth
以-y结尾的基数词改-y为-ie再加-th，如：twenty—twentieth, thirty—thirtieth
几十几的序数词只在个位数体现，如：twenty-one—twenty-first,thirty-three—thirty-third
特殊的序数词：one—first, two—second, three—third, nine—ninth
2. 不定冠词a与序数词连用序数词前加冠词a表示"再一次"。例如：Can I try a second time? 我可以再试一次吗?
3. 序数词用于分数中描述分数时，分子用基数词，分母用序数词，分子大于1时，分母加s。例如：1/5：one fifth 2/3：two thirds
1.A+be动词/谓语动词+倍数+as+形容词/副词原级+as+B例句：Women working as housekeepers in the under 35 age group in 2014 were almost twice as many as in 2004.
译文：2014年35岁以下的女性做保姆的人数几乎是2004年的两倍。
2.A+be动词/谓语动词+倍数+形容词/副词比较级+than+B例句：This building is three times higher than that one.
译文：这座建筑的高度是那座的四倍。(这座建筑比那座高三倍。)
3.A+be动词+倍数+the size/height/length/width，etc+of+B例句：This street is four times the length of that one.
4.times as many/much/often/比较级+as+被比对象例句：Young people used the Internet more than three times as often as the general population in 2014.
译文：在2014年，年轻人上网人数是一般大众的3倍多。
5.times+that+of+被比对象例句：In this workshop, the output of July was 4.5 times that of January.
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