It is a complicated tense for English speakers to come to terms with, the only way around it is to study the past tenses. Imperfect, Cuando era joven. = when I was young. Preterite, Ayer fui a Madrid. The preterite is used for a completed past action(s). The imperfect, to describe something, or what you used to do, or what you were doing when something interrupted you. Example, Ayer fui a Madrid, yesterday I went to Madrid. De niño todos los fines de semana iba a Madrid. as a child I used to go to Madrid every weekend. Happy studying

I think that the imperfect is used to an action developed along the time in the past. But this amount of time is indeterminate. The preterite is used to an action developed in the past but in a determinated time or date. You know when this started and when finished. It's difficult to explain. I thing that the best is read and understand examples. And remember, "fui" is the preterite of the verb "ir" too.

First, you need to understand it with the "normals" verbs, then you can understand it with "ser y estar" Imperfect is an "relative tense" and preterit is an "absolut tense". Imperfect: past in the time you're talking about. "había mucha gente en la fiesta cuando llegamos" ....but you don't know, don't explain, if there were "after and before" Imperfect: past ....absolutely and finished. "En la fiesta hubo mucha gente"