So basically, I'm aware in spoken German varieties the perfect tense is fairly dominant outside of some largely fixed verbs that are preterite dominant in the past tense. However, I'm after reading in this grammar book I have that there also exists in some usages a difference of tense/aspect between them.
Basically the book says that in situations where a progressive sentence would be used in the past (eg. I was going, I was eating, I was reading) the preterite often gets used instead of the perfect. So I'm basically asking to what extent this actually happens in spoken and written German and what are some examples.