Imperfect is a past tense form. It is something that happened in the past and continued to happen for a long time, vs preterit, which is something that happened in the past and ended. The imperfect does not necessarily have an end. For example, cuando era niña, me gustaba los animales. (When I was little, I liked animals.) liking animals happened for a long time, so I use imperfect. Cuando era niña me caí. (When I was little, I fell down). I fell down once, and I am not still falling down, so I use preterit.