To decide between when to use the pretérito perfecto and the pretérito, we need to keep in mind the unit of time in which an action took place and the unit of time in which the speaker is referring to it. From these two tenses, the pretérito perfecto is the closest to the present time. We use the pretérito perfecto with time expressions such as hoy, esta mañana, este mes, etc., to emphasize that an action is still inside the unit of time of the present. On the other hand, the pretérito refers to actions outside the unit of time, and therefore, this tense uses time expressions such as ayer, la