Guardamos la notificación de la multa por si dudamos.

Because Spanish usually drops subject pronouns when the verb already shows who is doing the action.

• guardamos already means we keep / we save
• dudamos already means we doubt / we are unsure

So Nosotros guardamos la notificación... is possible, but it is only used if you want extra emphasis, contrast, or clarity.

For example:

• Nosotros guardamos la notificación, pero ellos la tiraron.
  = We kept the notice, but they threw it away.

Not usually. In this context, guardar means to keep, to save, or to put away, not to guard in the English sense of protecting something like a security guard.

Common meanings of guardar include:

• guardar un documento = to keep/save a document
• guardar una copia = to save a copy
• guardar dinero = to save money
• guardar algo en un cajón = to put something away in a drawer

So here guardamos la notificación means something like we kept/saved the notice.

Formally, guardamos can be either:

• present: we keep / we are keeping
• preterite: we kept / we saved

That is a normal ambiguity in Spanish for many -ar verbs in the nosotros form.

In this sentence by itself, many learners will naturally read it as present, especially because dudamos is also present. But context decides.

Examples:

• Guardamos la notificación por si dudamos luego.
  = We keep the notice in case we have doubts later.
  → present

• Ayer guardamos la notificación.
  = Yesterday we kept/saved the notice.
  → preterite because ayer makes the time clear

Spanish often uses articles where English may not.

Here:

• la notificación = the notice / the notification
• de la multa = of the fine

This makes it sound like a specific notice connected to a specific fine.

So la notificación de la multa is a very natural noun phrase meaning the notice about the fine or the fine notice.

If you remove the articles, it would usually sound less natural or more abstract in this kind of sentence.

Por si is a very common fixed expression meaning in case.

It is used when you do something now as a precaution because something might happen later.

So the structure is often:

• hacer algo + por si + possible situation

Examples:

• Llévate un paraguas por si llueve.
  = Take an umbrella in case it rains.
• Guarda el recibo por si lo necesitas.
  = Keep the receipt in case you need it.

In your sentence, the idea is: we keep the fine notice as a precaution, in case uncertainty comes up later.

Because por si normally takes the indicative, not the subjunctive.

That surprises many English speakers, because in case often refers to something uncertain. But in Spanish, after por si, the usual pattern is:

• por si + indicative

Examples:

• por si viene
• por si llueve
• por si tienes dudas

So:

• por si dudamos = correct
• por si dudemos = not natural here

A helpful way to think about it: por si introduces a possible circumstance you are preparing for, not a wish or a purpose that would normally trigger the subjunctive.

Here dudamos means something like:

• we have doubts
• we are unsure
• we start doubting
• we are uncertain

The verb dudar often means to doubt or to be unsure.

In this sentence, the exact thing being doubted is not stated explicitly, but the context suggests something related to the fine or the notice, such as:

• whether the fine was correct
• what the notice said
• whether something was paid
• whether there was a deadline

So dudamos is understandable, but it is a little open-ended unless the context has already made clear what the doubt is about.

Yes, it can. Spanish allows dudar without an explicit object or clause when the context makes it clear.

So si dudamos can mean if we have doubts / if we are unsure.

That said, native speakers often make it more specific, for example:

• por si dudamos de algo = in case we have doubts about something
• por si tenemos dudas = in case we have doubts

In everyday Spanish, tener dudas is often a little more natural than bare dudar in this kind of sentence.

It is understandable, but many native speakers would probably say it a bit differently.

The part that can sound slightly less idiomatic is por si dudamos, because it feels somewhat incomplete or abstract on its own.

More natural alternatives might be:

• Guardamos la notificación de la multa por si tenemos dudas.
• Guardamos la notificación de la multa por si luego dudamos de algo.
• Guardamos la notificación de la multa por si acaso.

These sound a bit more everyday and explicit.

So the original sentence is not wrong, but it may feel slightly less common than those alternatives.

Yes. Por si acaso is a very common expression meaning just in case.

So:

• Guardamos la notificación de la multa por si acaso.

This sounds very natural and idiomatic.

Difference in nuance:

• por si dudamos = specifically in case we have doubts
• por si acaso = just in case, more general

So if you want to mention the exact possible problem, use por si + clause. If you want a broader precaution idea, por si acaso works very well.

Yes. The most natural place for por si... is often at the end, as in your sentence, but other orders are possible for emphasis.

For example:

• Guardamos la notificación de la multa por si dudamos.
• Por si dudamos, guardamos la notificación de la multa.

Both are understandable. The first is more neutral and more common in everyday speech. The second puts more emphasis on the precaution idea.

Because Spanish often expresses this kind of relationship with de + noun.

So:

• notificación de la multa = notice of the fine / fine notice

This is a normal way to say that the notification is connected to the fine.

English often compresses nouns more easily, as in fine notice or parking ticket notice, but Spanish usually prefers a structure with de:

• el recibo del pago
• la fecha del examen
• la notificación de la multa

So this is a standard Spanish noun construction.