I've written words that I never knew were in the world这句话的句子

I've written words (that I never knew were in the world).主谓宾结构.
I've written words是本句的主要部分,主语I,谓语 have written,宾语 words.
that (I never knew) were in the world是定语从句.其中I never knew是插入语,在不在都不影响句子语法结构,只会对意思有影响.
再问： 插入语可以直接放在这么？我的意思是不加标点符号或者其他标注么？I never knew和先行词没什么关系么？多谢你！！
再答： 有些插入语和先行词有关系例如本例，有些没有，例如口语爱说的i think, I mean之类的，其实插入语是一种口语化，非正规用法，所以不能全用语法判断，要用语义去分析。 标点貌似只有那种和先行词没有关系的插入语才需要加，而且不加往往问题不大。