- Did you find out who was behind that action?
- Did you find out who had been behind that action?
So Past Perfect refers to events that happened before the main event. So i think 2 is correct but 1 is used rather than 2. Then why is 1 correct?
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If you use the past perfect "Did you find out who had been behind that action", you are choosing to refer to the events relative to some focus point in the past.
I emphasise "choosing", because in many cases this is a completely free choice.
Sometimes, if you don't, clarity will be compromised because the sequence of events is important, and might not be clear.
But often, it makes no difference to the clarity whether you bring in that temporal focus or not. And indeed, native speakers often do not bother with the past perfect when the meaning is clear without it.