The research is so designed that once (begun) nothing can be done to change it.括号里的词为什么...

问题描述:

The research is so designed that once (begun) nothing can be done to change it.括号里的词为什么...
The research is so designed that once (begun) nothing can be done to change it.括号里的词为什么用过去分词?

research 被动begin:that once (省略了the research is) begun nothing can be done to change it.