A caução do apartamento é alta, mas a inquilina já a pagou.

In this sentence, caução means a security deposit paid for a rental property.

In Portugal, caução is a very natural word in housing and rental contexts. It refers to money given as a guarantee in case of damage, unpaid rent, and so on.

It is a feminine noun, which is important for words that agree with it later in the sentence.

Do is the contraction of de + o.

So:

• de = of / for
• o apartamento = the apartment
• do apartamento = of the apartment / for the apartment

In English, we would usually not translate it literally as of the apartment. More natural English would be:

• the apartment deposit
• the deposit for the apartment

Because alta agrees with a caução, and caução is feminine singular.

In Portuguese, adjectives usually agree in gender and number with the noun they describe.

So:

• o valor é alto = the amount is high
• a caução é alta = the deposit is high

Since caução is feminine, the adjective must be alta.

Here Portuguese uses ser because it is describing the amount/level of the deposit as a characteristic: it is high.

So:

• A caução é alta = the deposit is high

Using estar here would sound less natural in this context. Estar is more about temporary states or conditions, while ser is the normal choice for this kind of description.

Mas means but.

It is different from mais, which means more.

So:

• mas = but
• mais = more

This is a very common pair for learners to confuse because they look similar.

Yes. Inquilina is the feminine form of tenant.

• inquilino = male tenant
• inquilina = female tenant

So this sentence is specifically talking about a female tenant.

In Portugal, you may also see more formal or legal words such as arrendatário / arrendatária, but inquilino / inquilina is easy to understand and common in everyday language.

Because Portuguese uses definite articles very naturally with nouns.

Here:

• a caução = the deposit
• a inquilina = the tenant

In this sentence, both refer to specific things already identified in the situation: a particular deposit and a particular tenant.

For an English speaker, this often feels normal here because English would also use the. But in Portuguese, article usage is generally broader and more frequent than in English.

Já means already.

So:

• já a pagou = has already paid it / already paid it

It shows that the action happened before now, often with the idea of sooner than expected, by this point, or as a completed fact.

The a is a direct object pronoun meaning it.

It refers back to a caução.

So instead of repeating the noun:

• A caução do apartamento é alta, mas a inquilina já pagou a caução.

Portuguese replaces the repeated noun with the pronoun:

• A caução do apartamento é alta, mas a inquilina já a pagou.

Because caução is feminine singular, the pronoun is a.

This is an important European Portuguese point.

In European Portuguese, object pronouns often come after the verb in affirmative sentences:

• pagou-a = paid it

But certain words can trigger the pronoun to come before the verb. Já is one of those common triggers.

So in this sentence:

• já a pagou = natural and correct

This is a feature of pronoun placement in Portuguese called proclisis.

A useful contrast is:

• Pagou-a. = She paid it.
• Já a pagou. = She has already paid it.

Because Portuguese often uses the simple past where English prefers the present perfect.

So:

• já a pagou

can naturally mean:

• she already paid it
• she has already paid it

In English, has already paid it is usually the most natural choice here because the result matters now: the deposit is already paid.

Yes, absolutely.

That version is also correct:

• A inquilina já pagou a caução.

The original sentence uses a to avoid repeating a caução, which makes the sentence sound smoother and more natural.

So the difference is mainly stylistic:

• já pagou a caução = repeats the noun
• já a pagou = replaces it with it

Both are correct.