Normally, por is required when introducing a duration. I'm trying to understand why para can be used in certain contexts. For example:
- Una reserva por tres noches.
- Una reserva para tres noches.
- Nuestra ruta por los próximos días.
- Nuestra ruta para los próximos días.
Butt & Benjamin say, *"Por and para are interchangeable in time expressions fixing the duration of some future need."
But I'm wondering if anyone has any insight into why?
It's interesting that Butt & Benjamin give this as an example:
Necesito el coche para tres días.
But the half-dozen native speakers that I polled (Colombia, Mexico, & Spain) all preferred por in this case, and most flagged para as incorrect. Some of the Spaniards said para was technically correct, though they wouldn't use it.