I agree that you have too many words. Let's choose between "I don't know who is the person" and I don't know who the person is.Could you tell me please which words order is correct?
I don't know who is the person who said it.
I don't know who the person is who said it.
Maybe the difference is:" I don't know who said it." is how I'd put it. The extra words are unnecessary and awkward.
There is no difference in the meaning. The second one just has a lot of extra and unnecessary words to say exactly the same thing. No native English speaker would say it unless possibly for emphasis, e.g. I don't know WHO is the person that said it.Maybe the difference is:
I don't know who said it - I haven't heard about/seen/know this person.
I don't know who is the person that said it - I know who said it but I don't know what kind of man that person is.
None of them mean that. They all mean simply "I don't know who said it"
E.g.Why would you want to use this unidiomatic way of speaking?
Use I don't know who said it or the redundant variant I don't know who the person who said it is.
Fine.A teacher heard that a pupil just now said a dirty word. She turns to the class and asks: "Who said that? Mike, maybe it's you who did it?" "No, it wasn't I, and I don't know who said it."
The phrase "who wrote it" is unnecessary. You could also simply say, "I don't know who he is."Another case:
A teacher: "Who's written this dirty word on the blackboard?" A pupil: "It
'swas a boy. He entered the classroom, wrote this word and left the classroom." "Who is that boy?" "I don't know who that boy who wrote itis."