Using para with duration of a future need

Question

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.

Answer 1

Necesito el coche para tres días

I don't know how to sustain this, but that just doesn't sound quite right for me. Maybe it's regional, as you suggest.

By the other hand, I don't think "por" and "para" are interchangeable in time expressions. At list not in all cases. For example, this two sentences have a different meaning, in my opinion:

1. Necesito dinero para 3 días.

2. Necesito dinero por 3 días.

With the first one, I understand that the person needs money in a quantity that is enough to survive/buy things for 3 days. So, for example the person usually needs $100 by day then the person needs $300.

With the second one, the idea I have is that the person needs money, and if he/she is borrowing money, he/she is saying that the money will be returned within 3 days, which is not the same meaning as phrase 1.

Answer 2

Por shows a duration or span of time, often replacing durante, while para expresses goal or intent. In some cases either one can be grammatically correct and even interchangeable but, as observed by those people you polled, sometimes one of the two does not sound right.

Una reserva por tres noches sounds OK; it means "a reservation lasting three nights", i. e. three nights worth of a hotel room or the like.

Una reserva para tres noches is immediately understandable but does not sound OK; in this context one would have expected personas, not noches, since para indicates a goal, and the goal of the reservation is to get a room for one or more people. When ordering a room I'd say for example: Quiero hacer una reserva para dos personas por tres noches.

Nuestra ruta por los próximos días sounds OK: it means "our route (the route we'll be following) during the next few days". It conveys a sense of inevitability, not of planning. The emphasis is on the fact that you'll be going down a certain path for several days.

Nuestra ruta para los próximos días sounds OK as well, but here I'd say it means "our route (planned) for the next few days". The fact that you use para emphasizes intent: you're not simply following a path for some days, but you actually intend to do so. It's not the going that matters, but the fact that you intend to fill those next few days that way.

The distinctions can be subtle but I don't think it's correct to say, like Butt & Benjamin, that por and para are freely interchangeable in this context.