An opinion is what someone believes is true but has not been proved 分析句子结构

整个句子为主、系、表结构：An opinion /is /what someone believes is true but has not been proved.

这个句子比较复杂的是表语，是一个表语从句（也属于名词性从句）。因为从句的宾语语中缺少成分而是用"what"引导。

如果将WHAT放回从句中，从句正常的语序是：someone believes/ "what" is true but has not been proved. 从句谓语是believes，后面是一个宾语从句，含有两个并列句，但主语都是what，因此but后面省略了主语。（what)has not been proved是一个被动句.

不知我解释的明白否？