1.
what do you mean by saying that?
翻译：你说这话什么意思啊?
这个是一个固定的句式,没有太多道理,记住就好了.
2.
he has devoted his whole life to benefiting mankind.
翻译：他将他的一生献给了造福人类.
这里考察devote oneself to doing 的用法,所以benefit要用现在分词形式benefiting,不能用benefited.
望采纳,谢谢~~