过去分词在什么时候用??

过去分词可以做很多句子成分,比如说定语\状语(表被动完成)以及构成时态
例如:
i have finished my homework.(构成时态)
seen from the top of the hill, the city looks very beautiful.(过去分词作状语,表被动)
the boy called Tom is very brave.(过去分词作定语,表被动)