Se sobrar arroz, eu vou guardar na geladeira.

Because after se meaning if, Portuguese often uses the future subjunctive when the situation is still possible or uncertain in the future.

So in Se sobrar arroz..., sobrar is functioning as the future subjunctive, not as a simple infinitive in meaning.

This is especially confusing because for many -ar verbs, the future subjunctive looks exactly like the infinitive:

• sobrar → future subjunctive: sobrar
• guardar → future subjunctive: guardar

Compare with verbs where the form is different:

• Se eu fizer = If I do / if I make
• Se ele puder = If he can

So Se sobrar arroz means If there is rice left over.

In standard Portuguese, after se meaning if in this kind of future condition, you normally do not use the future indicative.

So these sound wrong or unnatural:

• Se sobrará arroz
• Se vai sobrar arroz

Instead, Portuguese uses the future subjunctive:

• Se sobrar arroz, eu vou guardar na geladeira.

This is one of the big differences from English. English says if there is or if there will be, but Portuguese has its own pattern:

• Se + future subjunctive
• then a main clause, often with a future idea such as vou guardar

Because arroz here is being used in a general, indefinite sense: if there is any rice left over.

Portuguese often leaves out the article with mass nouns when speaking generally or nonspecifically:

• Tem pão? = Is there any bread?
• Comprei leite. = I bought milk.
• Se sobrar arroz... = If there is rice left over...

If you say o arroz, it sounds more specific, like a particular rice that both speaker and listener already have in mind.

Yes, but the meaning shifts slightly.

• Se sobrar arroz = if there is any rice left over / if rice is left over
• Se sobrar o arroz = if the rice is left over / if that specific rice remains

With o arroz, you are pointing to a more definite, identifiable rice, such as the rice served at this meal.

In everyday speech, Se sobrar arroz is very natural when you mean leftover rice in a general sense.

Both are possible, but vou guardar is much more common in everyday Brazilian Portuguese.

• Eu vou guardar = I’m going to store it / I’ll store it
• Guardarei = I will store it

The simple future (guardarei) is correct, but it can sound more formal, more written, or less conversational. In spoken Brazilian Portuguese, people very often prefer:

• vou + infinitive

So this sentence sounds natural and conversational:

• Se sobrar arroz, eu vou guardar na geladeira.

No. Portuguese often drops subject pronouns when the verb already makes the subject clear.

So both are natural:

• Se sobrar arroz, eu vou guardar na geladeira.
• Se sobrar arroz, vou guardar na geladeira.

Including eu can add a bit of emphasis, contrast, or clarity, but it is not required.

It can be said, but it does not have to be.

In Se sobrar arroz, eu vou guardar na geladeira, the thing being stored is understood from context: it is the leftover rice.

So Portuguese can omit the object when it is obvious:

• Se sobrar arroz, vou guardar na geladeira.
• understood: vou guardar o arroz

If you want to make it fully explicit, you can say:

• Se sobrar arroz, eu vou guardar o arroz na geladeira.

That is grammatical, but a little more repetitive.

A very natural alternative would also be:

• Se sobrar arroz, eu vou guardar ele na geladeira.
  In informal Brazilian Portuguese, ele can refer to things, although this is more colloquial.

Na is a contraction of:

• em
  • a = na

So:

• na geladeira = in the refrigerator / in the fridge

This kind of contraction is extremely common in Portuguese:

• em + o = no
• em + a = na
• de + o = do
• de + a = da

In Brazil, geladeira is the usual everyday word for fridge/refrigerator.

Here sobrar means to be left over, to remain, or to be extra after use/eating.

So:

• Se sobrar arroz = If there is rice left over

It does not mean that the rice itself is doing the action in an active sense. It is more like:

• some rice remains
• some rice is left over after the meal

This verb is very common with food:

• Sobrou pizza. = There was pizza left over.
• Se sobrar bolo, me avisa. = If there’s cake left over, let me know.

Yes, a little.

The original sentence is very natural:

• Se sobrar arroz, eu vou guardar na geladeira.

You could also hear:

• Se sobrar arroz, vou guardar na geladeira.
• Eu vou guardar na geladeira se sobrar arroz.

But the original structure is especially common because Portuguese often puts the condition first:

• Se..., then the result

That makes the sentence easy to follow:

1. condition: Se sobrar arroz
2. result: eu vou guardar na geladeira

Yes, it fits Brazilian Portuguese very well.

The grammar is standard Portuguese, but some choices are especially typical of Brazil:

• vou guardar instead of a simple future like guardarei
• geladeira as the everyday word for fridge

A speaker from Portugal would understand it perfectly, but they might choose slightly different everyday wording depending on the situation.