主变被解题步骤:
1. 找宾语 ----即动作的承受者
They make shoes in that factory.
Shoes
2. 判断宾语的单复数 ----即be动词的单复数.
are were
3. 判断动词的时态 ----即be动词的时态.
4. 修改谓语的时态 ----即原句动词改为过去分词
made
5. 修改原句的主语 ----即by+ 主语/ 宾语.
by them.