Si la inquilina deja el piso limpio, le devolverán la fianza sin problema.

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

So:

• Si la inquilina deja el piso limpio... = If the tenant leaves the flat clean...
• ...le devolverán la fianza = ...they will return the deposit to her

This is a very common pattern:

• Si estudias, aprobarás = If you study, you will pass
• Si llegamos pronto, cenamos allí = If we arrive early, we’ll eat there

If the sentence were hypothetical or unlikely, then Spanish would change both parts:

• Si la inquilina dejara/dejase el piso limpio, le devolverían la fianza

That means something more like If the tenant were to leave the flat clean, they would return the deposit.

Devolverán is the simple future of devolver, so it means they will return.

It fits because the sentence describes a future consequence:

• If the tenant leaves the flat clean, they will return the deposit

Yes, Spanish could also use the present tense in some contexts:

• Si la inquilina deja el piso limpio, le devuelven la fianza

That can sound a bit more like a general rule or routine.
Using devolverán sounds a little more clearly future-oriented and slightly more explicit.

Inquilina means female tenant.

It is feminine because the sentence is referring to a woman:

• la inquilina = the female tenant
• el inquilino = the male tenant

In English, tenant does not change form for gender, but Spanish nouns often do.

In Spain, piso very often means flat/apartment.

So here:

• deja el piso limpio = leaves the flat clean

Although piso can also mean floor depending on context, in a housing/rental sentence from Spain, piso is very naturally understood as flat/apartment.

This is a useful Spain-specific vocabulary point:

• Spain: piso
• Many Latin American countries: apartamento or departamento

Because limpio agrees with el piso, not with la inquilina.

• el piso is masculine singular
• therefore the adjective is limpio

So the structure is:

• dejar + object + adjective
• dejar el piso limpio = to leave the flat clean

It does not mean that the tenant is clean. It means the flat is clean.

Compare:

• La inquilina está limpia = The tenant is clean
• La inquilina deja el piso limpio = The tenant leaves the flat clean

Because le is the indirect object: to her.

The thing being returned is la fianza, which is the direct object.

So:

• devolver algo a alguien = to return something to someone
• devolverán la fianza a la inquilina = they will return the deposit to the tenant

When you replace a la inquilina, you get:

• le devolverán la fianza

So:

• le = to her
• la fianza = the deposit

If you said la devolverán, that would mean they will return it, where la replaces la fianza, not the tenant.

In Spanish, the third person plural is often used when the specific people are not named.

So devolverán literally means they will return, but it often really means something like:

• the landlord
• the landlady
• the agency
• whoever is responsible

Spanish does this a lot when the exact subject is unimportant or obvious from context.

So this sentence does not need to say exactly who will return the deposit.

Here, fianza means a security deposit or rental deposit.

In rental contexts in Spain, la fianza is the money paid at the start of the tenancy and returned at the end if everything is in order.

So in this sentence:

• le devolverán la fianza = they will return her deposit

Be aware that fianza can have other meanings in other contexts, such as bond or bail, but in a housing sentence like this, deposit is the natural meaning.

Sin problema means without any problem, no problem, or with no issue.

It is very common in the singular as a set expression. Spanish often uses the singular in cases where English might use singular or plural depending on the wording.

So:

• sin problema = no problem / without any problem
• sin problemas = without problems

Both can exist, but sin problema is extremely natural and idiomatic here.

In this sentence, it adds the idea that the deposit will be returned easily or without difficulty.

Because the conditional clause comes first:

• Si la inquilina deja el piso limpio, le devolverán la fianza sin problema.

When the if clause comes before the main clause, Spanish normally uses a comma, just like English often does.

If the order is reversed, the comma is usually not needed:

• Le devolverán la fianza sin problema si la inquilina deja el piso limpio.

Both versions are correct; the first one simply starts with the condition.

Not exactly word-for-word, but that is often the natural idea.

Literally, dejar el piso limpio means to leave the flat clean.

That structure focuses on the resulting state:

• the flat ends up clean

In practice, in a tenancy context, it usually implies that the tenant has cleaned it properly before leaving.

So depending on context, a natural English translation might be:

• If the tenant leaves the flat clean...
• If the tenant leaves the apartment clean...
• If the tenant cleans the flat and leaves it in good condition...

The Spanish expression is about the final condition of the property.

Grammatically yes, but in Spain, piso is much more natural and common for a flat/apartment.

So for Spain Spanish:

• piso is the most idiomatic choice

If you said apartamento, people would understand you, but it may sound less neutral or sometimes suggest a holiday apartment or a different register, depending on context.

Since this is specifically Spanish from Spain, piso is exactly what you would expect.

Yes, it sounds natural and real.

It works well in everyday spoken or written Spanish, especially in discussions about renting. It is fairly neutral in tone:

• not especially casual
• not especially formal
• very normal and practical

A landlord, agent, or tenant could easily say or write something like this.