When do I use Pretérito imperfecto and when do I use Pretérito indefinido?

Pretérito imperfecto is a past tense that is used for routines, recurring activities, descriptions of situations and ongoing activities that were suddenly interrupted. For example: Cuando era niña, siempre jugaba con mis hermanos. (When I was a little girl, I always played with my brothers - recurring past activity)
Pretérito indefinido is equivalent to the English past simple tense. It is used for completed actions in the past. Key words are ayer (yesterday), pasado (last, for example la semana pasada - last week), en ... (in). For example: Ayer mi padre salió a las tres. (Yesterday my dad left at three o' clock).