I like to differentiate preterite imperfect by comparing it to a picture. You have foreground images and you have background images. The foreground images catch the eye and are more prominent, which makes them preterite. The background images are not the main focus but they add details that could be necessary/helpful to the overall theme.
Another way to differentiate is to look at actions and determine if they "add on" or if they just give background information." If they add on, the motion is adding new and necessary steps that continue the story. (preterite). If it's background information, the movement does not continue, but it just gives background knowledge
Era un día lluvioso y frío (no motion)
Eran las diez de la mañana (no motion)
Aunque llevaba (no motion), yo fui a caminar (starting the motion).
Yo saqué mi parabrisas (motion) caminé por una milla (motion) y entré en una tienda (motion)- also a succession of events.
^Here's a brief example of what I'm referring to. You can also paint a picture in your head to determine what would be happening in the foreground vs. the background.