【英语】耐心好心的People,come in 3

【英语】耐心好心的People,come in 3
Finshed their work,they wrote a paper about this work.
这里Finshed 过去分词省主语
化为 They finshed their work,they wrote a paper about this work.
这样子对吗?
对的话Finshed 过去分词在这里做什么语啊?
化为As they finshed their work,they wrote a paper about this work
做原因状语不行吗？