¿Por qué no quieres ir a su fiesta? - Porque no.Why won't you go to her party? - Just because.
b.
Una frase idiomatica cuyo significado no es deducible si se traduce palabra por palabra.
sin traducción directa
No me vengas con excusas. No hiciste la tarea porque no te dio la gana.Don't come to me with excuses. You didn't do your homework just because you didn't feel like it.
No hice la compra porque no tuve tiempo.I didn't do the shopping because I didn't have time.