yanx 1. I had thought that he had died at least 20 years ago. You are confusing present perfect with past perfect.
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yanx1. I had thought that he had died at least 20 years ago.You are confusing present perfect with past perfect. This sentence contains the past perfect, so the rules about the use of the present perfect don't apply.
yanx2. I have given up that idea long ago.This is wrong. It should be I gave up that idea long ago.