In Catalan, we use the "Imperfet" for past actions not concluded or things that we used to do. Example: Quan era petita anava a casa dels meus avis quan acabava l'escola. (When I was a child I used to go to my grandparents house when I finished school).
We use the "Passat (passat simple)" for past concluded actions. Example: Finalment, acabí la meva tasca. (Finally I ended my work).
I must say, that "Passat simple" is normally used only in written Catalan or in very formal contexts. When speaking, we use "Passat simple perifràstic". It is build with "anar" in present as an auxiliary verb + infinitive: Acabí (passat simple) = vaig acabar. Exemple: Finalment, vaig acabar la meva feina.
Hope it will help you! :-)