Pretérito Perfecto vs Pretérito Indefinido
Pretérito Perfecto: It is used to talk about an action that happened in the recent past
Este fin de semana he comprado un vestido
(This weekend I have bought a dress)
Pretérito Indefinido: It is used to talk about an action that started and finished in the past and it has no relationship with the present.
El fin de semana pasado compré un vestido
(Last weekend I bought a dress)