the past is hidden somewhere earlier this sentence. For example: Yesterday I saw Mike near the office again. He had not written a line since he arrived. so actually we have he hadn't written before I saw him. and [arrive] and [not write] are consequent actions

Hi, Good question. In your original sentence, “He had not written a line since he arrived,” the action of arriving comes first (in simple past), and the lack of writing continues afterward. We use “had not written” (past perfect) to show the longer or earlier time frame, and “arrived” (simple past) as the reference point. Your version, “He didn’t write a line since he had arrived,” is not wrong, but it’s less common and sounds awkward. The original version is clearer and more natural.

"He had not written a line since he arrived" is correct because: "arrived" = simple past (the reference point). "had not written" = past perfect (shows inaction from that point onward). Your version — "He didn't write a line since he had arrived" — is awkward and uncommon. Normally, "since" is followed by simple past, not past perfect.

Two completed actions in the past require the past perfect. "Had I known about the party, I would have gone."