first
second
third
fourth
fifth
sixth
seventh
eighth
ninth
tenth
eleventh
twelfth
thirteenth
fourteenth
fifteenth
sixteenth
seventeenth
eighteenth
nineteenth
twentith
以此类推
很简单的,1作个位除11外加ST,2作个位除12外加ND,3作个位加RD,其余都加TH
序数词就是排序,就象中文的第一,第二....一样,当你想说第几时就用序数词,其他情况就用基数词