El arrendatario va a desocupar el piso.

Not usually. Arrendatario is correct, but it sounds formal, legal, or administrative.

In everyday Spanish in Spain, people more often say inquilino.

So:

• arrendatario = tenant, in a contract/legal sense
• inquilino = tenant, more everyday/common

This means the sentence sounds a bit like something from a contract, notice, or formal conversation.

El is the masculine singular definite article, meaning the.

So:

• el arrendatario = the tenant
• el piso = the flat/apartment

Spanish uses articles very often, sometimes more often than English does. In this sentence, both nouns are specific: it is talking about a particular tenant and a particular flat.

Va a desocupar is the near future construction in Spanish:

• ir
  • a
    • infinitive

Here:

• va = he/she/it goes (from ir), but in this structure it helps form the future
• a desocupar = to vacate

So va a desocupar means is going to vacate.

This is extremely common in Spanish and often feels more natural in speech than the simple future:

• va a desocupar = is going to vacate
• desocupará = will vacate

Both are possible, but va a + infinitive is very common for plans or expected future actions.

Because the structure is fixed:

• ir + a + infinitive

You cannot normally say va desocupar.

You need the a:

• va a desocupar
• vamos a salir
• voy a llamar

In this pattern, a does not mean to in the same way it does in English word-for-word. It is just part of the Spanish future construction.

Here, desocupar el piso means to vacate the flat/apartment, to move out of it, or to leave it empty.

It is built from the idea of occupying something:

• ocupar = to occupy
• desocupar = to unoccupy / vacate / empty

In this sentence, it does not mean that the tenant is cleaning it or freeing up a little space. It means they are leaving the property.

That said, desocupar can sound a bit formal here. In everyday speech, Spaniards may more often say:

• dejar el piso
• irse del piso
• mudarse

depending on the situation.

In Spain, piso is the normal word for a flat/apartment.

So in Spain:

• piso = the usual word for an apartment/flat
• apartamento = often used for a holiday apartment, tourist apartment, or sometimes a smaller apartment

So el piso sounds very natural in Peninsular Spanish.

If you are learning Spanish from Spain, piso is the word you will hear all the time.

It is grammatical and correct, but it sounds somewhat formal.

Why?

• arrendatario is formal/legal
• desocupar is also a bit formal in this context

So the whole sentence feels like something from:

• a rental contract
• a legal notice
• a property management message
• a formal report

A more everyday version might be:

• El inquilino va a dejar el piso.
• El inquilino se va del piso.
• El inquilino va a mudarse.

So yes, the sentence is correct, but its register is not especially casual.

Then the noun and article change to match gender and number.

Singular feminine

• La arrendataria va a desocupar el piso.

Plural masculine or mixed group

• Los arrendatarios van a desocupar el piso.

Plural feminine

• Las arrendatarias van a desocupar el piso.

Notice that the verb also changes:

• va for singular
• van for plural

A rough pronunciation guide is:

el ah-ren-da-TA-rio ba a deh-soh-koo-PAR el PEE-so

A few useful notes:

• rr in arrendatario is a rolled/trilled r
• va a is often said smoothly together, almost like ba-a
• in most of Spain, z and soft c are pronounced with a th sound, but there is no z or soft c in this sentence
• piso in Spain is pronounced with a clear s sound

The stress falls on:

• arren-da-TA-rio
• deso-cu-PAR
• PI-so

Usually not in exactly the same way.

Desalojar often means to evict, to clear out, or to remove people from a place, often by an outside force. It can sound stronger and more official.

For example:

• La policía desalojó el edificio. = The police cleared the building.
• El casero desalojó al inquilino. = The landlord evicted the tenant.

But in your sentence, the meaning is that the tenant is going to leave the flat. For that, desocupar works, though it is formal. In everyday language, dejar el piso or irse del piso would often sound more natural.