stolen
过去分词做定语表被动：被偷的自行车为什么不填werestolen一个句子只能有一个谓语，后面were found是谓语了，所以前面不能有谓语，除非用从句：
The things that were stolen last week were found by the policeman。
过去分词做被动实际上就是上述从句的省略。