How do I use the imperfect and preterite tenses together to describe a completed event?

Well, let's start with an example. ej. Estaba atravesando la calle y de repente un coche se estralló contra un árbol. (ex. I was crossing the road and all of a sudden a car crashed into a tree.)This is a typical example of using the imperfect tense to set the scene, i.e. I was crossing the road, and the preterite tense to describe the main event, i.e. a car crashed.You could also think about it as a theatre performance. The imperfect tense would be the stage props and background actors providing the context and the preterite tense would be the protagonist and their actions.