Both preterite and imperfect are forms of the past tense. However, when talking about a single action that has been completed, the preterite is the appropriate tense. Imperfect is commonly used when discussing a recurring past event. So, in English, that would be like, "She used to run every day." Imperfect is used here since it was not a single action.
The sentence "We went to the mall" would translate into "Fuimos al centro comercial." Fuimos is the preterite conjugation of "ir" in the nosotros form.
