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序数词的构成

(1)一般在基数词后加th

eg.four→fourth,thirteen→thirteenth

(2)不规则变化

one→first, two→second, three→third, five→fifth,eight→eighth,

nine→ninth,twelve——twelfth

(3)以y结尾的十位整数，变y为ie再加th

twenty→twentieth, forty→fortieth, ninety→ninetieth

(4)从二十一后的“几十几”直至“几百几十几”或“几千几百几十几”只将个位的基数词变为序数词。

twenty-first,two hundred and forty-fifth