kind of ironical that the word for "simple" is pretty long and complex...
UK native speaker — Initially, I wanted to write "Quite simply, I do not know why" , which seems to me to be more natural than "Simply, I do not know why". Indeed, it is not something that I would ever say. I decided to use, "I simply do not know why" only because, as I have learnt from painful experience, I suspected that that was what Duolingo would expect.
(American English speaker) I did that, too, and got it wrong. I think the reason is that "simply" modifies the verb, "do not know." In Italian the pronoun is included in the verb so it is not an issue.
Nor should it be! Phrases are ordered differently in English and Italian. You--cannot--translate---word---for--word. Why don't people get that? "Simply I do not know why" is simply NOT said.
I had been translating "perché" as "why" when it was in a question, and "because" when it was in a statement. So, I translated this sentence as "because I simply do not know".
I believe I am mistaken here and need to refine my understanding of when "perché" means "why" and when it means "because" -- I think the fact that "perché" is at the end of the sentence here is a clue?
It's a good clue. Italian word order is flexible, but they wouldn't put "because" at the end of a "because" clause.