He has written and spoken widely on what he says are several mistaken beliefs that stop sb from doing sth.
……what he says are several …… 这里的are加上是什么意思？没懂

总的来时个that从句吧我认为。he says are severl mistakes应该是用来做what的定语吧- -与on形成介宾短语作主句的宾补。瞎猜的。！仅供参考。

从He has written and spoken widely on what he says到这里，是一个主语从句，来作句子中的主语。
are 就是这个句子中的系动词。
several mistaken beliefs that stop sb from doing sth.是充当这个句子的表语成分。
这是一个主语+系动词+表语 的句子结构，只不过每个成分都是由句子来充当，所以你看着比较复杂。