Se o elevador não funcionar, vou subir as escadas.

Because after se meaning if, Portuguese often uses the future subjunctive when talking about a possible future situation.

So:

• Se o elevador não funcionar... = If the elevator doesn’t work / if the elevator happens not to work...

This is different from English, where we usually use the present form after if.

Compare:

• Se ele vem is generally not correct for this kind of future condition.
• Se ele vier = If he comes
• Se o elevador não funcionar = If the elevator doesn’t work

For regular -ar verbs, the future subjunctive often looks the same as the infinitive, which is why you see funcionar.

Vou subir is the very common ir + infinitive future, literally I am going to go up, but in natural English it usually just means I’ll go up.

So:

• vou subir = I’m going to go up / I’ll go up

Portuguese also has a simple future:

• subirei = I will go up

Both are correct, but in everyday Brazilian Portuguese, vou subir is usually more natural and more common in speech.

Portuguese often drops subject pronouns when the verb already makes the subject clear.

• vou already tells you it means I go / I’m going / I will
• so (eu) vou subir is fine either way

In this sentence, leaving out eu sounds perfectly natural:

• Se o elevador não funcionar, vou subir as escadas.

If you add eu, it may give a little more emphasis or clarity:

• Se o elevador não funcionar, eu vou subir as escadas.

Portuguese uses definite articles very often, sometimes more often than English.

Here:

• o elevador = the elevator
• as escadas = the stairs

This sounds natural because the speaker is referring to a specific elevator and the stairs in that building or situation.

English sometimes leaves things more general, but Portuguese often prefers the article when the thing is understood from context.

Não normally goes directly before the verb it negates.

So:

• funcionar = to work / to function
• não funcionar = not to work

In the clause:

• Se o elevador não funcionar = If the elevator doesn’t work

That word order is completely standard in Portuguese.

It can literally mean to function, but in everyday Portuguese it is very commonly used where English would simply say work.

So:

• O elevador não funciona. = The elevator doesn’t work.
• Meu celular não está funcionando. = My phone isn’t working.

Even though function exists in English, work is often the more natural translation.

Yes. Subir as escadas is a normal and natural way to say go up the stairs or take the stairs.

Literally, it is:

• subir = to go up
• as escadas = the stairs

So the whole phrase means:

• to go up the stairs

Depending on context, English might translate it as:

• I’ll go up the stairs
• I’ll take the stairs

All of those fit the Portuguese sentence well.

Yes, and that changes the nuance slightly.

• Se o elevador não funcionar focuses on whether it works or not
• Se o elevador não estiver funcionando focuses more on the state of it not being in operation at that moment

Both are correct, but:

• Se o elevador não funcionar is shorter and very natural
• Se o elevador não estiver funcionando can sound a little more specific or immediate

So both work, but the original sentence is a very standard choice.

Because the sentence begins with a conditional clause:

• Se o elevador não funcionar = the if clause
• vou subir as escadas = the main clause

In Portuguese, when this kind of subordinate clause comes first, a comma is normally used before the main clause.

So the punctuation here is standard:

• Se o elevador não funcionar, vou subir as escadas.

Yes, elevador is the standard word in Brazilian Portuguese for elevator.

For a British English speaker, this corresponds to lift.

So:

• American English: elevator
• British English: lift
• Brazilian Portuguese: elevador

In Portuguese, se can mean different things in different contexts, but here it clearly means if.

In this sentence:

• Se o elevador não funcionar... = If the elevator doesn’t work...

That is a conditional use.

Portuguese se can also appear in other structures, but in this example you should understand it simply as if.

You could say:

• Se o elevador não funcionar, subirei as escadas.

That is grammatically correct and means the same thing.

However, in modern spoken Brazilian Portuguese, this sounds more formal or literary than:

• Se o elevador não funcionar, vou subir as escadas.

So learners will hear vou subir much more often in everyday conversation.