4-19基数词+th, fourth, fifth, sixth, seventh, eighth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth, fifteenth, sixteenth, seventeenth, eighteenth, nineteenth
第几十都是tieth结尾:twentieth, thirtieth, fortieth, fiftieth, sixtieth, seventieth, eightieth, ninethieth
第几十几:只需个位数变序数词
twenty-first, twenty-second, twenty-third, twenty-fourth...