基数词twenty-one的序数词 1. one ; first 2. two; second 3. three; third 4. four; fourth 5. five; fifth 6. six; sixth 7. seven; seventh 8. eight; eighth 9. nine; ninth 10. ten; tenth 11. eleven; eleventh12. twelve; twelfth 13. thirteen; thirteenth 14. fourteen; fourteenth 15. fifteen; fifteenth 16. sixteen; sixteenth 17. seventeen; seventeenth 18. eighteen; eighteenth 19. nineteen; nineteenth 20. twenty; twentieth 第21 twenty-first 21st 第22 twenty-second 22nd 第25 twenty-fifth 25th 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 本文来源：https://www.wddqw.com/doc/093ba7155427a5e9856a561252d380eb629423f0.html