⑤其他注意事项
1000以上的数字，从后向前数，每三位加“，”；第一个“，”前为thousand，第二个“前为million，第三个“，”前为one billion（美国）或one thousand million，然后每一节按百、十、个的顺序表示。 hundred,thousand,million,
billion作确定的数目讲时不用复数形式，如：two hundred两百，three thousand三千。若其后加名词时，名词需用复数，如：five hundred students 五百个学生。
基数词的用法
基数词起名词或形容词的作用，在句中作主语、宾语、表语、定语和同位语。 ①作定语
There are seven boats in the lake. 湖里有七条小船。 ②作主语
Five of them are girls and four are boys. 他们中五个女孩，四个男孩。 ③作动词宾语或介词宾语
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-How many hamburgers do you want? 你要几个汉堡包？ -Iwant five，please. 我要五个。
She's a mother of two . 她是一位有两个孩子的母亲。 ④作表语
Three and four is seven. 3加4是7。 ⑤作同位语
Is there room for us three?有我们三个人的位置吗？ 序数词
序数词的构成
序数词除了开头的三个词(first, second, third )有独特的形式外，其他都是在基数词词尾加-th构成。 基本序数词的构成
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1st first 2nd second 3rd third 4th fourth 5th fith 6th sixth 7th seventh 8th eighth 9th ninth 10th tenth 11th eleventh |12th twelfth 13th thirteenth 14th fourteenth |15th fifteenth 16th sixteenth
17th seventeenth 18th eighteenth 19th nineteenth 20th twentieth 21st twenty-first 22nd twenty-second 23rd twenty-third 24th twenty-fourth 25th twenty-fifth 26th twenty-sixth 27th twenty-seventh 28th twenty-eighth 29th twenty-ninth 3oth thirtieth 40th fortieth 50th fiftieth
60th sixtieth 7oth seventieth 80th eightieth 90th ninetieth 100th one hundredth 1000th one thousandth 1000 000th one millionth 1 000 000000th one billionth ①1-19的序数词的表达
1-3的序数词是独立单词，4-19的序数词在相应的基数词后加th，但第五fifth,第八
eighth,第九ninth，第十二twelfth，稍有差别，拼写时要注意。 ②20-90整十数的序数词的表达
以y结尾的基数词变序数词，变y为ie再加th构成。如： twenty-twentieth；thirty- thirtieth；ninety-ninetieth。
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