
Originally Posted by
PROESL
It's true they're at different times, but that does not make "I thought I replied" incorrect. It's sufficiently obvious that "the reply" came before the thought without forcing the use the use of the past perfect in such as a sentence as this. Or this is sufficiently obvious: First the speaker replied, or did not reply, and then the speaker thought he or she replied. The simple past still clearly tells us this. It's correct to use the past perfect as well, but not incorrect to use the simple past.
What you are referring to is the past perfect. The term pluperfect is rather antiquated, I would say.
The "past perfect" is not a tense. It's an aspect of the past, or a way in which we can talk about the past in a specifc way.