In the sentence "Éramos nosotros quienes llamamos a la puerta" (It was us who knocked on the door), the use of the imperfect tense "éramos" is appropriate because it sets the scene and describes an ongoing or repeated action in the past.

The imperfect tense is commonly used when narrating background information, describing habitual actions, or providing context for a specific event. In this case, "éramos" conveys that, at the time of knocking on the door, the action of being there and potentially considering whether or not to knock was ongoing or part of a larger context.

If you were describing a single, completed action of knocking on the door, you might use the preterite tense "fuimos" (we went) instead. However, in this sentence, it seems like the focus is on the state of being (éramos) when the action of knocking occurred. And besides, the example appears in a conjugation lesson on the imperfect past tense.

So, the use of "éramos" here suggests a more nuanced portrayal of the situation, emphasizing the ongoing presence or consideration before the specific act of knocking on the door.

Saludos

Silvia