过去分词作状语题目

过去分词作状语题目
1._Seen_ (see) from the top of the hill,the city looks beautiful to us.（帮我解释下）
2.The teach buildings _built_ (build) in 1960 need repairing.（为什么不是were built）
3.I found my school bag _left_ (leave) in Mary’s room.（为什么不是had left）
4._Lost_ in thought,he almost ran into the car in front of him.
5__Having_suffered__ such heavy pollution already,it may now be too late to clean up the river.(这句是不是因为两个句子之间构成时间上的先后,所以要用过去完成时?）
6.Don’t use words,expressions,or phrases _known_ only to people with specific knowledge.（why?）