序数词是数词的一种，主要在英语语法中讲到，在汉语中表示为“第几”。
1st = first2nd = second3rd=third4th=fourt，5th=fifth6th=sixth7th=seventh8th=eighth。9th=ninth10th=tenth11th=eleventh12th=twelfth，13th=thirteenth14th=fourteenth15th=fifteenth16th=sixteenth。
17th=seventeenth18th=eighteenth19th=nineteenth20th=twentieth。 21st=twenty-first22nd=twenty-second23rd=twenty-third24th=twenty-fourth。
25th=twenty-fifth26th=twenty-sixth27th=twenty-seventh28th=twenty-eighth。
29th=twenty-ninth30th=thirtieth。
