Hi there! Normally, the pretérito imperfecto is used to describe a usual action in the past whose end is not clear or is simply not indicated. For example: "Cuando yo era pequeña tocaba el piano". The main difference between pretérito imperfecto and pretérito indefinido is the end of the action; the pretérito indefinido is used to describe a completed past event or situation. For example: "Esta mañana saludé a mi vecino". A comparison with the two tenses: "María estudiaba Psicología" / "María estudió Psicología": in first case, you just mind the act of study, not the end of the action, whereas in second case the act is clearly finished. However, this is a very basic explanation:) Regards!

iduno

You should buy a grammar book and you will find your answer.