I alway confuse preterit and pluperfect.A friend reminded me last week in a reply letter in english.
I looked at an english grammar today,it say the main differentia are:
1.It exists time adverbial after the action in the preterit sentence,but no time adverbial at place in the pluperfect sentence.
2.We concern the result or effect of the action mainly in the pluperfect sentence but action itself in the preterit sentence.