Si la cerradura vuelve a fallar, la inquilina llamará a la dueña antes de acostarse.

Because after si for a real, possible condition, Spanish normally uses the present indicative, not the future.

So the usual pattern is:

si + present, + future / present / imperative

Examples:

• Si llueve, me quedaré en casa.
• Si tienes tiempo, llámame.

So in your sentence:

• Si la cerradura vuelve a fallar = if the lock fails again
• la inquilina llamará... = the tenant will call...

Using volverá after si would sound wrong in standard Spanish here.

Volver a + infinitive means to do something again.

So:

• volver a fallar = to fail again
• volver a intentar = to try again
• volver a llamar = to call again

Be careful: volver by itself usually means to return, but volver a + infinitive is a set structure meaning to do again.

Here fallar means to stop working properly, to malfunction, or to fail.

With machines, devices, parts, systems, etc., fallar is very common:

• El coche falla. = The car is malfunctioning.
• La conexión falla. = The connection is failing / acting up.
• La cerradura falla. = The lock isn’t working properly.

So this is not about a person failing in a moral or personal sense. It is about the lock not working.

Because it expresses the result of the condition: what the tenant will do if the lock fails again.

So the sentence has this structure:

• condition: Si la cerradura vuelve a fallar
• result: la inquilina llamará a la dueña

In English, this matches If X happens, she will do Y.

In everyday Spanish, you could also hear va a llamar:

• Si la cerradura vuelve a fallar, la inquilina va a llamar a la dueña.

That is more conversational, but llamará is completely correct.

That a is needed because llamar a alguien means to call someone, and Spanish uses the personal a before a specific person.

So:

• Llamo a María.
• Van a llamar al técnico.
• La inquilina llamará a la dueña.

Even though la dueña is the direct object, Spanish still uses a because it refers to a person.

Literally, la dueña means the female owner.

In this context, since there is la inquilina (the tenant), it most likely refers to the woman who owns the property, so in natural English it may well be understood as the landlady.

That said, in Spain you might also hear:

• la propietaria = the female owner / property owner
• la casera = the landlady

Dueña focuses on the idea of ownership.

Because after antes de, Spanish uses the infinitive when the subject is the same as the subject of the main clause.

Here, the main subject is la inquilina, and the person going to bed is also understood to be la inquilina.

So:

• antes de acostarse = before going to bed

The reflexive pronoun se attaches to the infinitive:

• acostar + se → acostarse

If the subject changes, then Spanish uses antes de que + subjunctive:

• La inquilina llamará a la dueña antes de que se acueste la dueña. = The tenant will call the owner before the owner goes to bed.

By default, it is understood to be the tenant: la inquilina.

So the sentence means:

• If the lock fails again, the tenant will call the owner before the tenant goes to bed.

This is because with antes de + infinitive, the infinitive usually refers back to the subject of the main clause unless context clearly suggests otherwise.

Because Spanish often uses the definite article with specific, known things or people.

Here, the sentence sounds like it is talking about:

• a particular lock
• a particular tenant
• a particular owner

So:

• la cerradura = the lock
• la inquilina = the tenant
• la dueña = the owner / landlady

This is very normal in Spanish. The article is often used whenever the speaker assumes the listener knows which person or thing is being referred to.

Yes. Spanish word order is fairly flexible.

For example, you could also say:

• Si vuelve a fallar la cerradura, la inquilina llamará a la dueña antes de acostarse.

That sounds natural too.

You can also put the si clause after the main clause:

• La inquilina llamará a la dueña antes de acostarse si la cerradura vuelve a fallar.

A useful punctuation point:

• If the si clause comes first, a comma is normally used.
• If it comes second, the comma is usually not used.

Because si without an accent means if.

Compare:

• si = if
• sí = yes

So in this sentence:

• Si la cerradura vuelve a fallar... = If the lock fails again...

No accent is needed.

Not exactly.

• acostarse = to go to bed / to lie down to sleep
• dormirse = to fall asleep

So:

• antes de acostarse = before going to bed not
• before falling asleep

That distinction matters. The tenant will call before going to bed, not necessarily before actually falling asleep.