'have forgotten' covers a period of time, from some point in the past, to NOW, the moment of speaking. How can I be saying 'I have forgotten all about it" at exactly the same moment, NOW, that I am obviously aware, by now remembering, that I have forgotten...see how complicated a paradox it is?
"I had totally forgotten that we had decided to meet at the beginning of each month. Should I assume that from now on we will meet/will be meeting on the first Wednesday of each month?"
Student or Learner