O carro está molhado por dentro, porque a janela ficou aberta.

Because estar is used for temporary states or conditions. Being wet is a condition, not a permanent characteristic of the car.

So:

• O carro está molhado = the car is wet
• O carro é molhado would sound wrong in normal usage

Portuguese uses definite articles more often than English. Here, the sentence is talking about a specific car and a specific window, so o and a are natural.

They also show grammatical gender:

• o carro = masculine
• a janela = feminine

This is grammatical gender. It usually has nothing to do with real-world sex or gender. Objects can be masculine or feminine simply because that is how the noun works in Portuguese.

So you learn them as:

• o carro
• a janela

That gender affects articles, adjectives, and some other words in the sentence.

Because adjectives in Portuguese must agree with the noun they describe.

• molhado describes o carro

  • carro is masculine singular
  • so it must be molhado

• aberta describes a janela

  • janela is feminine singular
  • so it must be aberta

It means on the inside or inside.

So O carro está molhado por dentro means that the interior of the car is wet, not the outside.

It is a very natural expression when talking about the inside part of something.

Sometimes dentro can work, but por dentro is more idiomatic here.

• por dentro focuses on the inside part of something
• dentro often focuses more on location

So molhado por dentro is the most natural way to say wet inside in this sentence.

Here ficar + adjective/past participle means something like:

• became
• ended up
• remained

So a janela ficou aberta means the window ended up open, remained open, or more naturally in English, the window was left open.

That makes it a very good explanation for why the inside of the car is wet.

Estava aberta just describes a past state: it was open.

Ficou aberta adds the idea that the window ended up in that state or was left that way. That nuance fits well here, because the sentence is giving a cause.

So:

• estava aberta = was open
• ficou aberta = ended up open / was left open

Because here it means because, introducing a reason.

• porque = because

This is different from forms like porquê, which means the reason.

In this sentence, porque is the correct form because the second clause explains the cause.

The comma creates a slight pause and makes the second part sound like an added explanation.

You will also often see this sentence without the comma:

O carro está molhado por dentro porque a janela ficou aberta.

That version is also natural. The comma is not changing the basic meaning; it mainly affects rhythm and style.

The difficult part is lh.

In Portuguese, lh is a single sound, similar to the lli in million for many English speakers, though not exactly the same.

A rough guide is:

• molhado ≈ mo-LYA-do

The important thing is not to pronounce it as separate l + h sounds.