"I have forgotten" refers to the present.
"I had forgotten" refers to the past.
Today, it suddenly occurs to me. Once, I started to remember it. It should become past already. So, just like "I had never thought of it before", once you thought of it, it become past already.
I do not know, am I right?
"Have forgotten" is something (forgot) in the past but is relevant to something taken place at the time of speaking, i.e. relevance;
"Had forgotten" is a completed past action occurred before another past action, which has nothing to do with anything taken place at the time of speaking, i.e. irrelevance.
(Not a teacher)