After a fight Hayate, Nagi heads to the apartment complex to hang out.  But her “little sister” is the only one home.  This might be Nagi’s chance to find out what Ruri’s real goal is. 
Why? Why do you have to go shopping now?
¿Por qué? ¿Por qué tienes 
que ir de compras ahora?
¿Qué? ¿Por qué te tienes que ir justo ahora?
Por quê? Por que 
você vai fazer compras?