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

It looks like an infinitive, but in this sentence it is functioning as the future subjunctive.

After se when you are talking about a future possibility, Portuguese normally uses the future subjunctive:

• Se sobrar feijão... = if there are beans left over...
• Se chover... = if it rains...
• Se ele vier... = if he comes...

With many regular -ar verbs, the future subjunctive has the same form as the infinitive, so sobrar and the infinitive sobrar look identical.

So in this sentence, se sobrar means something like if there ends up being some left over.

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

So:

• sobrar feijão = for beans to be left over
• se sobrar feijão = if there are any beans left over

A few similar examples:

• Sobrou comida. = There was food left over.
• Se sobrar bolo, eu como depois. = If there is cake left over, I’ll eat it later.

It is not the same as deixar or guardar.
Sobrar describes the food still remaining after people have eaten.

Because Portuguese often allows the subject to come after the verb, especially when introducing something indefinite or when the sentence means if any X is left.

So:

• Se sobrar feijão... sounds like if there are any beans left over
• Se o feijão sobrar... sounds more like if the beans are left over or if that specific bean dish remains

The version with feijão after the verb is very natural here because the speaker is talking about the possible existence of leftover beans, not emphasizing the beans as a known subject.

Without an article, feijão is being used in a more general or indefinite way:

• Se sobrar feijão... = if there are any beans left / if some beans are left over

With o, it becomes more specific:

• Se sobrar o feijão... = if the beans remain / if that specific bean dish is left over

In everyday Portuguese, when talking about leftover food in a general sense, the version without the article is very common and natural.

In Brazilian Portuguese, feijão is very often treated as a mass noun when referring to the food/dish, not to individual beans.

So:

• feijão = beans as food
• arroz = rice
• macarrão = pasta

English often uses a plural word here, but Portuguese usually uses the singular food name:

• comer feijão
• sobrou feijão
• vou guardar o feijão

If you wanted to talk about individual beans, you would normally use something more specific, such as grãos de feijão.

The pronoun eu is optional here.

Portuguese verbs already show the subject:

• vou guardar = I’m going to store it
• eu vou guardar = I’m going to store it

Including eu can add:

• emphasis
• contrast
• clarity in conversation

So both are natural:

• Se sobrar feijão, vou guardar na geladeira para amanhã.
• Se sobrar feijão, eu vou guardar na geladeira para amanhã.

The version with eu may sound a bit more explicit or slightly more emphatic.

Because ir + infinitive is extremely common in spoken Brazilian Portuguese for future meaning.

So:

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

Both are correct, but they differ in style:

• vou guardar = more natural in everyday speech
• guardarei = more formal, more written, sometimes more stiff in conversation

That is why eu vou guardar sounds very normal in Brazil.

Portuguese often leaves the direct object unstated when it is obvious from context.

Here, everyone already knows the thing being stored is feijão, so saying just vou guardar is perfectly natural.

Possible versions:

• Se sobrar feijão, eu vou guardar na geladeira... = very natural
• Se sobrar feijão, eu vou guardar o feijão na geladeira... = correct, but repetitive
• Se sobrar feijão, eu vou guardá-lo na geladeira... = grammatically correct, but more formal/literary in Brazil

In everyday Brazilian speech, omitting the object is often the most natural choice.

Because na is the contraction of:

• em + a = na

So:

• guardar na geladeira = store in the refrigerator

Other common contractions work the same way:

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

Since geladeira is feminine singular, em a geladeira becomes na geladeira.

Here guardar means something like store, put away, or keep.

In the context of food:

• guardar na geladeira = put/store in the fridge

So guardar is a very natural verb for putting leftovers away for later.

It can also be used in many other contexts:

• guardar dinheiro = save money
• guardar um documento = keep/store a document
• guardar um brinquedo = put away a toy

So the core idea is to keep something in a place for later use.

Here para amanhã means for tomorrow: the beans will be stored so they can be eaten or used tomorrow.

• guardar na geladeira para amanhã = store it in the fridge for tomorrow

If you wanted the idea until tomorrow, Portuguese would more naturally use:

• até amanhã

So:

• para amanhã = for tomorrow, intended for tomorrow
• até amanhã = until tomorrow

In this sentence, para amanhã is the natural choice because the speaker is saving the food to use the next day.

Yes. In spoken Brazilian Portuguese, pra is a very common contraction of para.

So both are natural:

• para amanhã = slightly more neutral/full form
• pra amanhã = very common in everyday speech

Likewise:

• para a geladeira can become pra geladeira in speech, depending on the sentence

In writing, para is a bit more standard, but pra is extremely common and normal in informal Brazilian Portuguese.

Yes, and it would sound natural, but the structure is slightly different.

Compare:

• Se sobrar feijão... = if there are beans left over / if any beans end up left
• Se tiver feijão sobrando... = if there are beans left over

Both work. The original sentence is a bit more direct and compact.
The tiver ... sobrando structure is also very common in Brazilian Portuguese and may sound a little more conversational in some contexts.

So the original sentence is perfectly natural, but it is not the only way to express the idea.