O sistema finalmente voltou.

O is the masculine singular definite article, meaning the.

So:

• o sistema = the system

Portuguese uses definite articles very often, and here it works just like English the.

Because sistema is a masculine noun in Portuguese.

Even though many words ending in -a are feminine, sistema is one of the common exceptions. It comes from Greek-origin patterns, and many nouns ending in -ma are masculine in Portuguese, such as:

• o problema
• o tema
• o programa
• o sistema

So the correct article is o.

Finalmente means finally.

In this sentence, it comes between the subject and the verb:

• O sistema finalmente voltou.

That placement is very natural in Portuguese.

You can also move it for emphasis, for example:

• Finalmente, o sistema voltou.
• O sistema voltou, finalmente.

All of these are possible, but O sistema finalmente voltou sounds very normal and neutral.

Voltou is the 3rd person singular of the verb voltar in the preterite (pretérito perfeito).

That means it describes a completed event in the past:

• voltar = to return / to come back
• voltou = returned / came back

Here it suggests that at some point the system was down or gone, and then it came back.

Portuguese often leaves out subject pronouns when the verb form already makes the subject clear.

So instead of saying:

• Ele voltou.

Portuguese can simply say:

• Voltou.

In your sentence, the subject is already explicitly stated as o sistema, so there is no need for ele.

Not necessarily. In Portuguese, voltar is used both literally and figuratively.

So O sistema finalmente voltou can mean things like:

• the system came back
• the system returned
• the system came back online
• the system started working again

The exact nuance depends on context. In tech or service situations, it often means the system is working again.

It could. That would be a slightly more explicit version:

• O sistema finalmente voltou a funcionar.
  = The system finally started working again.

But O sistema finalmente voltou is shorter and very natural when the context already makes it clear that the system had stopped working.

So:

• voltou = came back / returned
• voltou a funcionar = returned to working / started working again

Yes. That is correct and natural.

Compare:

• O sistema finalmente voltou.
• Finalmente, o sistema voltou.

Both are good. The difference is mostly one of emphasis and rhythm:

• O sistema finalmente voltou sounds like a neutral statement.
• Finalmente, o sistema voltou puts extra focus on finally, as if the speaker is relieved or impatient.

In Brazilian Portuguese, voltou is pronounced roughly like vol-TOH, with the stress on the last syllable.

A simple approximation:

• vol = like vol in volume, but shorter
• tou = roughly like toh with a light w/glide at the end

The stressed syllable is tou.

So the whole sentence is approximately:

• o sis-TEH-ma feen-al-MEN-chee vol-TOH

That is only an approximation for English speakers, but it helps with the stress pattern.

They are similar, but voltou is usually more common and more natural in everyday speech.

• voltou = came back / returned
• retornou = returned

Retornou can sound a bit more formal or technical depending on context.

For a sentence like this, especially in everyday Brazilian Portuguese, O sistema finalmente voltou sounds very natural.

Yes, very normal.

Portuguese is flexible with adverb placement, but putting the adverb before the verb is extremely common:

• O sistema finalmente voltou.

This is similar to English The system finally came back.

You will often see this pattern:

• subject + adverb + verb

For example:

• Ela já chegou. = She already arrived.
• O site ainda funciona. = The site still works.
• O sistema finalmente voltou. = The system finally came back.

Yes. Voltar is a very common verb with a broad meaning range, including:

• to return
• to come back
• to go back
• to turn back

Examples:

• Voltei para casa. = I went back home / I came back home.
• Ela voltou cedo. = She came back early.
• O sistema voltou. = The system came back.

The exact English translation depends on context, but the core idea is always returning.