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

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

• na: in
• amanhã: tomorrow
• ir: to go (future auxiliary)

Questions & Answers about Se sobrar peixe, eu vou guardar na geladeira para amanhã.

Why is it se sobrar and not se sobra?

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.

Why does the future subjunctive form sobrar look exactly like the infinitive?

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.

What does sobrar mean here?