Quando o elevador não funciona, eu preciso descer as escadas.

Portuguese often uses the definite article more than English does.

So o elevador literally means the elevator. In this sentence, it refers to the elevator in the building or the elevator being talked about in that situation.

In English, we might sometimes say when elevator service is down or when the elevator doesn’t work, but in Portuguese, o elevador sounds natural and expected.

In Portuguese, não usually goes directly before the verb it negates.

So:

• funciona = works
• não funciona = does not work

This is the normal way to make a sentence negative in Portuguese.

Yes, funciona comes from funcionar, which literally means to function, but in everyday English this is usually translated as to work.

So o elevador não funciona is the natural way to say:

• the elevator doesn’t work
• the elevator is not working

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

Here, quando is being used for a general or repeated situation: whenever / when the elevator doesn’t work.

In Portuguese, for habitual or general truths like this, the present tense is normal:

• Quando chove, eu fico em casa.
• Quando o elevador não funciona, eu preciso descer as escadas.

If you were talking about a specific future event, Portuguese often uses the future subjunctive instead:

• Quando o elevador não funcionar, eu vou descer as escadas.

So in your sentence, the present tense shows a usual situation, not one single future event.

Yes, it could.

Portuguese often drops subject pronouns when the verb already makes the subject clear. So both are correct:

• Eu preciso descer as escadas.
• Preciso descer as escadas.

Including eu can add emphasis, contrast, or clarity. Without special emphasis, many speakers would simply say preciso descer as escadas.

Preciso is the first-person singular present form of precisar.

So:

• precisar = to need
• preciso = I need

In this sentence:

• eu preciso descer = I need to go down

It is a very common and useful verb.

Because with an infinitive verb, Brazilian Portuguese usually says precisar + infinitive directly:

• preciso descer
• preciso sair
• preciso estudar

But with a noun, you normally use de:

• preciso de ajuda
• preciso de dinheiro

So:

• preciso descer = correct
• preciso de ajuda = correct

That is a very common pattern to remember.

Descer as escadas is a very common and natural way to say go down the stairs.

The verb descer already contains the idea of downward movement, so Portuguese does not need another verb like go.

Compare:

• descer as escadas = go down the stairs
• subir as escadas = go up the stairs

You may also hear other expressions, such as descer pela escada or ir pelas escadas, but descer as escadas is extremely standard and idiomatic.

Because Portuguese normally refers to stairs in the plural, just like English often does.

• as escadas = the stairs

The singular escada can mean staircase, stair, or even ladder, depending on context. But when talking about using the stairs in a building, as escadas is the most natural choice.

Yes. That would also sound natural.

Both express necessity:

• preciso descer as escadas = I need to go down the stairs
• tenho que descer as escadas = I have to go down the stairs

The difference is small:

• preciso often feels like need to
• tenho que often feels like have to

In everyday Brazilian Portuguese, both are very common.

Yes, it is natural.

A native speaker might also say:

• Quando o elevador não funciona, preciso descer as escadas.
• Se o elevador não funciona, eu preciso descer as escadas.
• Quando o elevador quebra, preciso descer pelas escadas.

But your original sentence is completely normal and clear.

The most likely small change in casual speech would simply be dropping eu:

• Quando o elevador não funciona, preciso descer as escadas.