1. first
基数词one对应的序数词为特殊变化,具体为first。
2. second
基数词two对应的序数词为特殊变化,故本题答案为:second
3. third
基数词three的序数词为特殊变化,故本题答案为:third。
4. fifth
基数词five,需变ve为f,在词尾+th变为相应序数词,故本题答案为:fifth.
5. ninth
基数词nine转换为序数词时,需去e,在词尾+th,故本题答案为:ninth.
6. twelfth
基数词twelve需变f为ve在词尾+th变序数词,故本题答案为:twelfth.
7. twentieth
基数词twenty需变y为i,在词尾+eth,变序数词,故本题答案为:twentieth.
8. twenty-first
基数词twenty-one 变序数词时只需变个位,故本题答案为:twenty-first.