DL suggests two correct solutions:
We'd opened a door. We opened the door.
I wrote "We opened a door". Is something wrong with this answer? Why can we open "the door" but not "a door" here?
It has to be a missing translation, from Polish side there is no way of saying if this is "a door" the door", "doors" or "the doors", although plural without clarification is less likely.
also otworzyliśmy can be "have opened", "had opened", or "opened"
But maybe there is an English rule.