Spanish imperfect vs. preterite?

It's not easy to describe or understand, but I ure you, the more you use Spanish, the easier it will become for you to get the feel of the usage. I walked to school. Good, done, over with. But, as I was walking to school... there was a girl driving by in a convertible. Yes, she already drove by but in the telling or retelling of the event, it is in the moment. Trying to view it strictly in English terms and rules, makes things less clear for you.