Sometimes I am not sure whether or not I need to use the past perfect or leave it out completely.
Why didn't he tell her what had happened?
Why didn't he tell her what happened? (Are both of these sentences correct)
He had fallen on the floor and then went to hospital as he had an injury.
He fell on the floor and then went to hospital as he had an injury.
I assume the first one suggests that it had taken some time for him to make his way to the hospital after falling, whereas the second one suggests that the it all happened quite fast.