Se o sistema do banco ficar fora do ar, eu vou pagar a fatura amanhã.

Because se is the conjunction meaning if.

• se = if
• si is normally used after prepositions, like para si (for oneself)

So in conditional sentences, Portuguese uses se:

• Se chover, eu fico em casa. = If it rains, I’m staying home.

After se referring to a future possibility, Portuguese normally uses the future subjunctive.

So:

• Se o sistema do banco ficar fora do ar...

Here, ficar is the future subjunctive form of ficar for ele/o sistema.

This is one of the most important patterns in Portuguese:

• Se eu puder... = If I can / If I’m able to...
• Se você tiver tempo... = If you have time...
• Se ele vier... = If he comes...

In English, we usually use the present after if. In Portuguese, future conditions often use the future subjunctive.

It looks like an infinitive, but here it is actually the future subjunctive.

For some verbs, the future subjunctive form is identical to the infinitive in certain persons. That is why it can be confusing.

With ficar:

• infinitive: ficar
• future subjunctive:
  • eu ficar
  • você/ele/ela ficar
  • nós ficarmos
  • vocês/eles/elas ficarem

Because the subject is o sistema, the correct form is ficar.

Ficar fora do ar is a very common expression meaning something is down, offline, out of service, or off the air, depending on context.

Literally:

• fora = outside
• do ar = of the air

But the expression is idiomatic. It is used for things like:

• websites
• banking systems
• TV or radio stations
• internet services
• apps or servers

So o sistema do banco ficar fora do ar means the bank’s system is unavailable or down.

Both are possible, but they give slightly different shades of meaning.

• estar fora do ar focuses on the state: the system is down
• ficar fora do ar often suggests becoming down or ending up down

In real everyday Brazilian Portuguese, the difference is often small, and both can work:

• Se o sistema do banco estiver fora do ar...
• Se o sistema do banco ficar fora do ar...

This sentence with ficar sounds very natural and common.

Do is a contraction of de + o.

• de = of / from
• o = the
• de + o = do

So:

• o sistema do banco = the bank’s system / the system of the bank

Portuguese uses these contractions all the time:

• da = de + a
• dos = de + os
• das = de + as

Portuguese usually expresses this kind of relationship with de rather than by stacking nouns the way English often does.

English:

• bank system
• credit card bill
• company policy

Portuguese often prefers:

• sistema do banco
• fatura do cartão
• política da empresa

So o sistema do banco is the normal Portuguese structure.

Both are grammatically correct, but eu vou pagar is much more common in everyday Brazilian Portuguese.

• eu vou pagar = near future / spoken, natural, very common
• eu pagarei = simple future, more formal, more written, sometimes more emphatic

So:

• Se o sistema do banco ficar fora do ar, eu vou pagar a fatura amanhã.

sounds very natural in Brazil.

Portuguese uses definite articles more often than English does.

So where English may say:

• pay the bill or even in some contexts just
• pay my bill

Portuguese often says:

• pagar a fatura

The article a is normal here. It does not necessarily need extra emphasis. It just sounds natural.

Fatura usually means bill, invoice, or very often credit card bill/statement, depending on context.

In Brazil, pagar a fatura often strongly suggests paying a credit card bill, though it can also be understood more generally depending on the situation.

Other related words:

• conta = bill/account, often for utilities or restaurant bills
• boleto = a specific type of payment slip used in Brazil
• fatura = invoice/statement, especially credit card contexts

Because adverbs of time are flexible in Portuguese, and putting amanhã at the end sounds very natural.

You could also hear:

• Amanhã eu vou pagar a fatura.
• Eu vou pagar a fatura amanhã.

Both are correct. The end position is very common and neutral.

Because the sentence begins with a conditional clause:

• Se o sistema do banco ficar fora do ar, ...

When this kind of dependent clause comes first, Portuguese normally separates it with a comma before the main clause.

So the structure is:

• Se + condition, result

Like:

• Se chover, eu não saio.
• Se você puder, me liga.

Because the main clause is stating the result in a normal future form, not another subordinate clause that requires the subjunctive.

Structure:

• Se o sistema do banco ficar fora do ar = condition, using future subjunctive
• eu vou pagar a fatura amanhã = result, using a future expression in the indicative

This is the standard pattern for a real or possible future condition in Portuguese.

Yes. Portuguese often drops subject pronouns when the meaning is clear.

So this would be completely natural:

• Se o sistema do banco ficar fora do ar, vou pagar a fatura amanhã.

Including eu can add clarity, contrast, or emphasis, but it is not required here.