Should I use the imperfect or perfect tense in this sentence?

The perfect tense refers to a completed event in the past where the effect of the event may still continue in the present. Whereas, the imperfect tense describes ongoing past events without reference to a time of starting or finishing. In narrative texts the imperfect usually provides a background of ongoing events. The imperfect tense can also refer to an habitual action in the past (in English we would translate as "used to" for this meaning). There is one exception to these rules where the imperfect tense can be used to describe completed events when a writer wants to make the past event more immediate but this is more of a minor point.