Se sobrar peixe, eu vou guardar na geladeira para amanhã.

Because this sentence is talking about a possible future situation: if there is fish left over.

After se in that kind of conditional sentence, Portuguese normally uses the future subjunctive:

• Se sobrar peixe, vou guardar... = if there’s fish left over, I’ll store it...
• Se sobra peixe, eu guardo... sounds more like whenever fish is left over, I store it or a habitual pattern.

So sobrar here is not the infinitive in function, even though it looks the same. It is the future subjunctive form.

That is normal for many verbs in Portuguese.

For regular -ar verbs, the future subjunctive in the eu/ele/ela/você form often looks exactly like the infinitive:

• infinitive: sobrar
• future subjunctive:
  • quando eu sobrar — no, this person form does not exist like that because the verb is normally used with things, not with eu
  • se ele sobrar
  • se sobrar peixe

A clearer full pattern is:

• se eu falar
• se você falar
• se eles falarem

So in se sobrar peixe, the form looks like the infinitive, but grammatically it is the future subjunctive.

Here sobrar means to be left over, to remain, or to be leftover.

So:

• Se sobrar peixe... = If there is fish left over...

It does not mean to spare here. The idea is that after eating, some fish may remain.

Because peixe here means fish in general / some fish / any fish left over, not a specific identified fish.

So:

• Se sobrar peixe... = if there is fish left / if any fish is left

If you said o peixe, it would sound more specific:

• Se sobrar o peixe... = if the fish is left over

That could work in some contexts, but the original sentence sounds more natural for a general food situation.

Also, like English fish, peixe can work as a food noun without an article.

No, it is not necessary.

Portuguese often drops subject pronouns because the verb form already shows the person:

• Se sobrar peixe, vou guardar na geladeira para amanhã.

This is very natural in Brazilian Portuguese.

Using eu adds a little emphasis, contrast, or simply makes the sentence slightly more explicit:

• Se sobrar peixe, eu vou guardar...

So both are correct.

Because in everyday Brazilian Portuguese, ir + infinitive is extremely common for the future.

So:

• eu vou guardar = I’m going to store / I’ll store
• guardarei = I will store

Both are grammatically correct, but vou guardar sounds much more natural in normal conversation.

This is one of the most common ways Brazilians talk about the future.

Because it comes after the conjugated verb vou.

In the structure ir + infinitive, the first verb is conjugated and the second stays in the infinitive:

• eu vou guardar
• ela vai comer
• nós vamos sair

So vou guardar literally works like I am going to store.

Guardar is a flexible verb. It can mean:

• to keep
• to store
• to put away
• sometimes to save

In this sentence, the most natural meaning is to store or to keep:

• vou guardar na geladeira = I’ll store it in the fridge

So this is not about guarding something like protecting it. It is about putting food away for later.

Because the object is understood from context: it is the fish.

So the full idea is:

• Se sobrar peixe, eu vou guardar [o peixe] na geladeira para amanhã.

In Brazilian Portuguese, it is very common to leave out a direct object when it is obvious. That sounds natural and avoids unnecessary repetition.

You could say:

• Se sobrar peixe, eu vou guardar ele na geladeira...

But that is less elegant here, and many speakers would simply omit the object.

Because na is the contraction of:

• em + a = na

And geladeira is a feminine noun:

• a geladeira = the refrigerator

So:

• na geladeira = in the refrigerator

Portuguese usually uses a preposition here where English just says in the fridge.

Here it most naturally means for tomorrow:

• vou guardar na geladeira para amanhã = I’ll keep it in the fridge for tomorrow

The idea is: I’m storing it now so that it can be eaten or used tomorrow.

If you specifically wanted the idea until tomorrow, Portuguese often uses:

• até amanhã = until tomorrow

So:

• para amanhã focuses on the intended use tomorrow
• até amanhã focuses more on the time limit

In many real situations, the meanings are close, but the nuance is different.

Yes. In spoken Brazilian Portuguese, para is very often reduced to pra.

So these are both natural:

• para amanhã
• pra amanhã

The full form para is a little more neutral or formal in writing, while pra is very common in everyday speech and informal writing.