Hi,

Could you please tell me what the difference is between had not and did not?

Eg-

My friend told me he had failed because he had not studied.

My friend told me he had failed because he did not study.

Moreover, would the reported speech for the following two sentences be the same-

Friend: I failed because I did not study / I failed because I had not studied.

Reported Speech: He said he had failed because he had not studied.

Is it ok to use past perfect twice in that sentence or any other sentence for that matter? Does it not confuse the reader?

eg-

Friend: I was disappointed that I was not allowed to resubmit my assignment.

Reported Speech: He said he had been disappointed that he had not been allowed to resubmit his assignment.

Can't you just write it in simple past?

Sorry for asking too many questions and thanks.
hammerman1I failed because I did not study / I failed because I had not studied.
The core meaning is the same. "Didn't" seems to sound more common.

hammerman1Friend: I was disappointed that I was not allowed to resubmit my assignment.
Reported Speech: He said he had been disappointed that he had not been allowed to resubmit his assignment.
Can't you just write it in simple past?
Yes, in fact I prefer simple past for this passage.

If we can deliver a context clearly without any chronological confusion, use simple tense. It makes life easier.

I was disappointed that I couldn't resubmit my ........
Thanks.