Hi Rusty,
Thank you for your detailed response with many example sentences!
I see that:
"Me calenté antes de que tú llegaste." & "Me había calentado antes de que tú llegaste." both convey the idea that "I" was finished warming up (in the context of exercise) prior to "You" arriving.
I guess the real difference between the sentences is a bit nuanced. It would be like me explaining the difference between "I warmed up before you arrived" and "I had warmed up before you arrived" to a person learning English.
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