In spanish, the imperfect tense is used to express past actions that have a definite end to express past habitual actions to express past background information to express uncertain future actions

the answer is number 1: to express past actions that have a definite end

when the outside temperature is too cold, an endothermic animal can generate heat by shivering. endothermic animals must eat much more often than ectothermic animals since it takes energy to maintain a constant body temperature.