Hola Laura,
I understand it is quite tricky because of the lack of Imperfect tense in English. We have two lessons about this difference, one, using time markers that trigger one or the other. In this lesson you will find more examples. Have a look here.
With the second lesson we did about this difference we wanted to show that even if there is no time phrase to trigger one tense or another, the speaker decides which tense to use depending on how he/she sees the action in his/her head: as a completed action with a beginning and an end, or as an action that simply happened but the "when" is not relevant.
I hope seeing those other examples in the lesson attached heps.
Saludos
Inma
