基数词+序数词:one[基数词].first〔序数词〕 two.second three.third four.fourth five.fifth six.sixth seven.seventh eight.eighth nine.ninth ten.tenth eleven.eleventh twelve.twelfth thirteen.thirteenth fourteen.fourteenth fifteen.fifteenth sixteen.sixteenth seventeen.seventeenth eighteen.eighteenth nineteen.nineteenth twenty.twentieth twenty-one.twenty-first twenty-two.twenty-second twenty-three.twenty-third twenty-four.twenty-fourth twenty-five.twenty-fifth twenty-six twenty-sixth twenty-seven.twenty-seventh twenty-eight.twenty-eighth twenty-nine.twenty-ninth thirty.thirtieth thirty-one thirty-first
基数词变序数词的规律是一般直接+th. ve结尾的变f再加th