13th=thirteenth
14th=fourteenth
15th=fifteenth
16th=sixteenth
17th=seventeenth
18th=eighteenth
19th=nineteenth
20th=twentieth
21th=twenty-first
22th=twenty-second
23th=twenty-third
24th=twenty-fourth
25th=twenty-fifth
26th=twenty-sixth
27th=twenty-seventh
28th=twenty-eighth
29th=twenty-nineth
30th=thirtieth
31th=thirty-first
总之,基数词变序数词多是加th就行了,不过在遇到y时,应把y改为i,再加-eth.如twenty变为序数词为twentieth.