Use the past perfect tense to refer to the event or action in the more distant past

When describing two events in the past that did not occur at the same time, use the past perfect tense to refer to the event or action in the more distant past.

Correct: I suddenly remembered that I had left my purse at the office.
Incorrect: I suddenly remembered that I left my purse at the office.

(Because leaving the purse at the office preceded remembering the fact, the past perfect form of had left should be used.)