When is the imperfect tense used and how is it formed?

The imperfect tense is used to describe recurring actions that have taken place in the past. In order to know whether or not to use the imperfect tense or the preterite tense, you can use the acronym SWEATER. S: scenery - background informationW: weatherE: emotions, A: ageT: time E: evolving or continuous actionsR: routine.There are only 2 sets of verb endings used in the imperfect tense, one set for AR verbs and one set for ER & IR verbs and are as follows;AR: aba, abas, aba, ábamos, abais, aban.ER/IR: ía, ías, ía, íamos, íais, ían.