I was supposed to answer the question "Did you post my letter?" I answered "No. I have forgotten to." It was a wrong answer. The right one was "No I forgot to."
Can I use Present Perfect there? On one hand since now I remember that I was supposed to post the letter, absent of this knowledge does not prolong up to now. But on the other hand, there is a consequence of my forgetfulness that is relevant now—the letter is not posted.