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
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