1.基数词变序数词。其规律为：
1,2,3特殊记，加th从4起 (first,second,third,fourth)
8少t，9去e，千万别忘记 (eighth,ninth) 逢5逢12，ve变f (fifth,twelfth)
20到90，y要变ie (twentieth,ninetieth) 若是几十几，前基后序别倒位 (ninety-first)
2.序数词常与定冠词the 连用。
练习:
①Henry has learned eight _________ French words this year.
red reds red of reds of
②The _________lesson is the most difficult one in Book Two. (twenty)
