序数词，表示顺序，第1，第2，第3，表示第几个
1-30的序数词，前3个是特殊情况，需要单独记忆，从4开始往后，基本上都是在基数词的词根后面加th，但是词根变化较多，尤其结尾部分，要注意 。
还有，所有序数词之前，都要加the 。
1-10,first, second, third,fourth,fifth,sixth,seventh,eighth,ninth,tenth
11-20,eleventh,twelfth,thirteenth,fourteenth,fifteenth,sixteenth,seventeenth,eighteenth,nineteenth,twentieth
21-30,twenty first, twenty second,twenty third,twenty-fourth,twenty-fifth,twenth-sixth, twenty-seventh,twenty-eighth,twenty-ninth,thirtieth
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