May be this website could help to answer your question: http://www.todoele.net/botiquin_mat/preterito_imperfecto.pdf In my opinion, the difference between pretérito imperfecto and pretérito indefinido is that the first one is used to describe an action that lasts for a period of time (for instance: "dormía"), while the second one is used to describe and action that started and finished, without the idea of a long duration (for instance: "durmió"). Do you see the difference?