As an English teacher, I often find students unclear about the use of the past perfect. It seems that this is sometimes optional if there is a time reference. I take both these sentences to be correct and mean the same:
Until I went to the Tower of London, I didn't believe in ghosts.
Until I went to the Tower of London, I hadn't believed in ghosts.
However, in these two examples, I don't believe they mean the same:
Until I went to London, I didn't eat sausages.
Until I went to London, I hadn't eaten sausages.
The first being a habitual act, or decision not to eat sausages; the second the absence of sausage eating during the speaker's lifetime, perhaps due to a lack of sausage-eating opportunities.
So, my questions are, can the past perfect be substituted by the past simple plus a time reference (or conjunction) only when a state verb is involved?
Are there any 'rules'(which time phrases or conjunctions can be used, for example)? I'm sure my students would be keen to know!