1到21的序数词

1-10 first second third fourth fifth sixth

seventh eighth ninth tenth

11-20 eleventh twelfth thirteenth fourteenth sixteenth seventeenth eighteenth nineteenth twentieth

21 twenty-first

拓展:基数词变序数词口诀: 一二三要全变，其余把th加后面，th里面有例外，八加h，九去e，ve要用f替整十基数变序数，变y为ie，再加th若是变为几十几，十位不变个位变