Hola Thea,
in the passive voice, the use of the preterite or the imperfect tense will depend on the action being seen as a completed action (preterite) or seen as a descriptive action (imperfect), so, for example:
1. Los ladrones fueron arrestados.
2. Los ladrones eran arrestados.
In 1., this is seen as a completed action, i.e. it has an end. In 2., we see it as a description of what was happening, as a process, not as a completed action.
I hope this clarified it.
Saludos,
Inma
