Student or Learner
I want to know if I should use a question mark or a period after a sentence like 'I forgot to ask you yesterday if you have heard back from Rich yet'. Also, should I use 'had' instead of 'have' in the clause to make it consistent with the past tense of the main sentence?
Use the question mark only when you ask a direct question.
Have you heard back from Rich yet?
When you have an indirect question, as you do here, use a period/full stop.
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.