Se o elevador parar de funcionar, vou descer as escadas.

Here se means if. It introduces a condition:

Se o elevador parar de funcionar... = If the elevator stops working...

So the whole sentence has a classic if X happens, I will do Y structure.

After se when you are talking about a possible future situation, Portuguese normally uses the future subjunctive.

So:

• Se o elevador parar... = If the elevator stops...

This is standard Portuguese for future conditions. With many verbs, the future subjunctive looks exactly like the infinitive, so parar may look strange at first. Compare:

• se ele fizer = if he does
• se ela for = if she goes/is
• se nós tivermos = if we have

So parar here is not the infinitive in function, even though it has the same form.

Parar de funcionar means to stop working or to stop functioning.

It is made of:

• parar = to stop
• de = of/from, but here part of the expression
• funcionar = to function, to work

So literally it is something like to stop functioning. This is a very common way to talk about machines, devices, appliances, elevators, cars, etc.

Because parar de + infinitive is the normal pattern for to stop doing something.

Examples:

• parar de fumar = to stop smoking
• parar de falar = to stop talking
• parar de funcionar = to stop working

If you just say o elevador parou, that means the elevator stopped, which could sound like it physically stopped moving. Parou de funcionar makes it clear that it broke down or stopped operating properly.

Vou descer is the very common spoken way to express the future in Brazilian Portuguese. It literally means I am going to go down, but in everyday use it often just means I’ll go down / I will go down.

So:

• vou descer = very common, natural in speech
• descerei = correct, but more formal or less common in everyday conversation

Both are grammatical, but vou descer sounds more natural in most spoken Brazilian Portuguese.

Because Portuguese often leaves out the subject pronoun when the verb already makes the subject clear.

• vou already tells you it is I
• so (eu) vou descer and vou descer both mean the same thing

Including eu is possible, but it usually adds emphasis or contrast:

• Eu vou descer, não você. = I’m going down, not you.

As escadas is a very natural way to say the stairs in Portuguese.

Portuguese can use both singular and plural in related ways:

• a escada = the staircase / the stairway / the ladder
• as escadas = the stairs

In this sentence, descer as escadas is the most natural equivalent of go down the stairs. Using the plural matches the idea of a set of steps.

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

Here:

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

These are specific, identifiable things in the situation: the elevator in the building, and the stairs available there. Using um elevador would sound like an elevator, which does not fit this context as well.

Yes, absolutely. That is fully natural.

Both are correct:

• Se o elevador parar de funcionar, vou descer as escadas.
• Vou descer as escadas se o elevador parar de funcionar.

When the if clause comes first, a comma is normally used. When it comes second, the comma is usually omitted.

Yes. A very common alternative is quebrar in the sense of break down:

• Se o elevador quebrar, vou descer as escadas.

That sounds very natural in everyday Brazilian Portuguese. The difference is:

• parar de funcionar = stop working, stop functioning
• quebrar = break, break down

Parar de funcionar is a little more neutral and explicit; quebrar is a little shorter and very common in speech.