Copreterite vs Past Progressive

When I first started learning Spanish using the Michel Thomas course it stuck with me that whenever you wanting to say in English; I was ..... We were..... You were to use the copretérito ie. -----ía + person or ----aba + person Now I am confused give that you can use estar + participle in the past tense i.e Also note that estar here is conjugated using the copreterit form too. estaba haciendo mis tareas estaba viendo la television So the question is how do I know when I should use the past progressive (if at all) as it almost seems as though the copreterite tense renders it obsolete. Thanxs In advance for any comments Sal2 Nev

You have to use the past progressive when you're talking about what you were doing when something happened. *Estaba cocinando cuando sonó el teléfono.* _I was cooking when the phone rang._ *Estaba hablando con Sarah cuando llegó Marco.* _I was talking with Sarah when Marco arrived._ Use the past progressive when you want to emphasize the continuous nature of what was happening. For example, *Lo estaba haciendo, pero tú me dijiste que tenía que hacer otra cosa.* _I was doing it, but you told me that I had to do something else._