Se la lifto ne funkcios, ni devos uzi la ŝtuparon.

Because the sentence is talking about a future situation: if the elevator does not work, then we will have to use the stairs.

In Esperanto, after se (if), you normally use the tense that matches the actual time of the action. So:

• Se la lifto ne funkcias = If the elevator isn’t working / doesn’t work now
• Se la lifto ne funkcios = If the elevator won’t work / doesn’t work in the future

Unlike in English, Esperanto does not usually avoid the future tense in if-clauses.

Devos is the ordinary future tense: will have to.

• ni devos uzi la ŝtuparon = we will have to use the stairs

Devus means would have to / should, and it sounds more hypothetical, conditional, or less direct.

So:

• Se la lifto ne funkcios, ni devos... = a real future possibility
• Se la lifto ne funkcius, ni devus... = a more hypothetical or imagined situation

Because ŝtuparon is the direct object of uzi.

In Esperanto, direct objects take -n:

• uzi la ŝtuparo → incorrect
• uzi la ŝtuparon → correct

Here, what will we use?
Answer: la ŝtuparon

That is why it gets the accusative ending.

Because la lifto is the subject of funkcios, not the object.

In the clause:

• la lifto ne funkcios

the elevator is the thing doing the action of functioning / working, so it stays in the normal subject form without -n.

Compare:

• La lifto funkcios. = The elevator will work.
• Ni uzos la lifton. = We will use the elevator.

In the second sentence, lifton is the object, so it gets -n.

Because ŝtuparo is a collective noun. It means a staircase, a stairway, or the stairs as a whole.

Related words:

• ŝtupo = a single step
• ŝtuparo = a staircase / stairway / stairs

So uzi la ŝtuparon is a perfectly natural way to say use the stairs.

English often uses a plural expression here, but Esperanto commonly uses the singular collective noun.

Because both are understood as specific things in the situation:

• la lifto = the elevator we are talking about
• la ŝtuparon = the stairs/staircase in that building or place

Esperanto uses la much like English the. In this sentence, it is not talking about just any elevator or any staircase, but the specific ones relevant to the situation.

Yes. Esperanto word order is fairly flexible.

For example, you could also say:

• Ni devos uzi la ŝtuparon, se la lifto ne funkcios.

This means the same thing: We will have to use the stairs if the elevator doesn’t work.

The original order is very natural because it presents the condition first.

Because it comes after devos.

Esperanto modal verbs like devi (must / have to), povi (can / be able to), and voli (want) are followed by an infinitive:

• ni devos uzi = we will have to use
• mi povas veni = I can come
• ŝi volas manĝi = she wants to eat

So uzi stays in the infinitive form.

Funkcii means to function, to operate, or to work.

So:

• La lifto funkcias. = The elevator works / is working.
• La lifto ne funkcios. = The elevator will not work.

It is a very common verb for machines, systems, plans, and methods.

Yes. Lifto is a normal Esperanto word for elevator / lift.

Esperanto often borrows international words and adapts them to its spelling and endings. Here:

• lift → lifto

Depending on region and style, learners may also encounter other words, but lifto is easy to understand and completely natural.

Ne usually goes directly before the word or part that it negates.

Here:

• la lifto ne funkcios

means the elevator will not work.

If you place ne before the verb, it negates the verb phrase. That is the most common pattern.

Not really in its current form. The future tense makes it sound like a specific future situation.

• Se la lifto ne funkcios, ni devos uzi la ŝtuparon.
  = a future case

For a more general meaning, Esperanto would more naturally use present tense:

• Se la lifto ne funkcias, ni devas uzi la ŝtuparon.
  = If the elevator doesn’t work, we have to use the stairs

So the tense choice helps show whether the statement is about a specific future event or a general situation.