Student or Learner
Sentence: "They were dead long time ago I'm/have been warning them about this from 12-Dec-2007 but they love to live in denial."
My question: Which grammar tense would be correct to use after the word ago "I'm" or "have been" ?
Note: I'm referring to some organization by the word "they" in my sentence and my sentence is in reply of one person who has also used the word dead for the same organization.
How do you warn dead people?
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.