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Past: I wrote something. I wrote it yesterday. Completed action, in the past.
Present perfect: I have written something I want you to read. The action is complete, but it connects to the present.
Past perfect. I had written something and shared it with a friend, and then I saw my writing with my friend listed as the author. I felt completely betrayed. -- Use this for a completed event in the past that comes before something else also in the past.