The παρατατικός (past continuous) emphasizes on the duration or on the repetition of an action in the past. That's why it is usually combined with words or expressions which show duration or repetition.
ex: “Εχθές διάβαζα όλο το απόγευμα”= Yesterday I was studding all evening. We emphasize on the duration.
“Πέρυσι ξυπνούσα κάθε πρωί στις 6.00.”= Last year I used to wake at 6.00 every morning. Here we emphasize on the repetition.
Ο Αόριστος (simple past) just refers to an action that happened in the past or used to happen, but without any emphasis to duration or repetition.
ex: “Εχθές διάβασα τα μαθήματά μου και μετά πήγα βόλτα”= Yesterday I did my homework and afterwards I went out.
Let's examine a phrase written in both variations:
Αόριστος: “Πέρυσι δούλεψα πολύ σκληρά.” = Last year I worked very hard. Here the emphasis is more on the quality of work (hard).
Παρατατικός: “Πέρυσι δούλευα πολύ σκληρά.” = Last year I was working very hard. Here we emphasize more on the fact that this was happening every day last year (duration and repetition).