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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