If the subject in the main clause expresses a past personal opinion, emotion, doubt, fear or hope about another subject (in the dependent clause) then the latter will be in a past subjunctive form—usually the congiuntivo imperfetto or the congiuntivo trapassato:
*Note that in this sentence you have two options (congiuntivo imperfetto or condizionale passato) because the action in the dependent clause is chronologically subsequent to that of the main clause.