He is always very brave when facing difficulties and danger. 这其实是个省略句. 完整的句子应该是这样的: He is always very brave when he is facing difficulties and danger. 前后主语一致,可以把后面的【主语+助动词】省略. “他面对”,是主动的,所以是facing而不是faced.
He is always very brave when facing difficulties and danger 意思是 当面对困难与危险的时候,他总是很勇敢。 用facing而不用face是因为在facing前面的单词是when。在这里的意思是“当”(当什么什么发生时) 所以face要改成动词形式。 懂了么?