What would help with some of the examples, to clarify why it is preterito that should be used, not imperfecto, is to give more context...

Question

For example: We were in Brazil with friends could easily also use estabamos, if it were the setting for when something happened:
Estábamos en Brasil con amigos cuando ella murió
La semana pasada, estuvimos en Brasil con amigos

Answer

Hola Sandy
As this lesson is a conjugation one, so we want to test the "form" not the "usage" we always give hints to which tense to conjugate. Some sentences that we've used have time frames but others don't as they are not essential.
When we make a lesson about when to use a specific tense or choosing between one tense or another you will always find a time frame to help you choose.
Gracias y un saludo