For example, a French's football gift is better than Z.Zidane, but he never do his best in matches, you can use "恨铁不成钢" 恨爹不成钢 is 恨爹不成刚, it's a scandal in China: a yong man drive his car too fast, made a girl die and another girl hurt terribly in an accident. He said to witness rampantly "Just take me to judge, my father is Li Gang(李刚), I won't be punished". Then people use 恨爹不成刚 to show the arrogant of the children of officials.

哈哈 看到外国人有这个问题我快笑死了 禁不住来说一下 这个就像典故一样，just like an old story ,只是这是由于去年发生的“我爸是李刚”网友根据“恨铁不成钢”恶搞的 “恨铁不成钢”came from a masterpiece in China,"恨爹不成刚”came from an event happened last year in China. “恨铁不成钢”原出自清朝曹雪芹《红楼梦》第九十六回：“只为玉玉不上进，所以时常恨他，也不过是“恨铁不成钢”的意思。”形容对所期望的人不争气不上进感到不满，急切希望他变好。 去年在中国轰动一时的“我爸是李刚”富二代开车撞人不悔过事件，网友用“恨爹不成钢”来形容希望自己的爸爸是李刚那样有权有势的人，实际是暗讽。 I do not know if I explain it clearly.

恨铁不成钢，to wish iron could turn into steel at once; to set a high demand on sb. in the hope that he will improve. We usually use this idiom in our daily life to express our feeling as disappointed. for example, He was expected to be a high-salary person by her folks, in fact, he is paid only less than $1000.His father always said,“to my son, I am always disappointed by him, actually, 恨铁不成钢."

恨跌不成钢 是儿子恨爸爸不成钢， 恨铁不成钢 是 爸爸、领导或者上级恨晚辈不成钢

生活中，常用 恨铁不成钢。而恨爹不成刚，则是网络新用语。在生活中不常用。