过去分词作定语时为什么没有be的出现？？？？

比如i heard that the boy （called ）tom had been chosen as monitor.还愿成定语从句是i heard that the boy（ who is called） tom had been chosen as monitor.在将定语从句转换为后置定语的时候定于从句的连接词和is要一起省略。如果还保留is的话一个句子就有两个动词了那么这个句子就是错误的句子了