The preterit and the imperfect are the 2 main Spanish past tenses.
For many Spanish learners they are a nightmare - mostly because we don't have exact equivalents in English grammar. That always makes it harder to learn because there isn't anything you can go off of in your native language to understand the second language.
But it doesn't have to be so difficult.
Differences between preterit and imperfect
Essentially, the preterit refers to a fixed, finished action in the past.
Does the action have a definite start/end? Use the preterit.
Are you referring to a specific point in time in the past when the action happened? Use the preterit.
The imperfect refers to an action in the past that continued on, set the stage for another event or one that didn't start and end at a specific time.
Era vs. Fue
This video on how to use ERA and FUE should give you a clearer idea. Of course, you could also enroll in our preterit vs. imperfect course at I Taught Myself Spanish Academy and get all of our other Spanish courses as well.
If you're interested in checking out the ESTUVO vs ESTABA video, you can sign up to view it here: