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.

　　 A.hundred B.hundreds C.hundred of D.hundreds of

　　 ②The _________lesson is the most difficult one in Book Two. (twenty)