要是句子的话 就是The letter was written to sb.
要是表示被动 则是the letter written to sb.
比如
But a letter written by Zhu pointed to what may have been an even deeper motive.
但是一封朱军生前所写的信或许能揭示更深层的原因。
Yesterday afternoon, I received a letter written to me by pupils from a primary school forwardedby a deputy.
就在昨天下午,我收到了一个代表转来的一所小学学生写给我的信。
追问"要是表示被动 则是the letter written to sb." 被动里反而没有be动词吗?
追答这里的被动是修饰letter的