Hola Esther

Here are some Kwiziq lessons that will help you understand the difference:

Ser in the imperfect vs the preterite

Estar in the imperfect vs the preterite

And here is another more general lesson on the use of these two tenses if you want to see more extended examples:

The imperfect vs the preterite (general use)

I hope you find them useful.

Saludos

Inma