All that a man has to say or do that can possibly concern mankind
显然,第一个that与all一起构成主语,a man has to say or do that can possibly concern mankind作定语,第二个that也引导定语从句,用来修饰说明前面的所有成分——All that a man has to say or do .
这句话的意思大概是：一个人所必须说和做的与人类可能相关的所有事情
后面所有的——is in some shape or other to tell the story of his love - to sing,and if he is fortunate and keeps alive,he will be forever in love就是谓语和宾语.