新概念二82课中：Such creatures have rarely been seen alive by man.

新概念二82课中：Such creatures have rarely been seen alive by man. 其中alive的用法
新概念二中有一句：Such creatures have rarely been seen alive by man as they live at a depth of six hundred feet.
这句话我有点糊涂：
1.alive在这里是形容词吗,属于什么用法?
2.这句话如果用主动句如何表达?
谢谢!
再问一个问题：这里man为什么用单数？是否可以用men?
谢谢！