O serviço militar terminou.

In Portuguese, it is very common to use the definite article with a noun when speaking about something specific or understood in context.

So:

• O serviço militar terminou. = The military service ended.

Without the article, serviço militar can sound more general or more like a label/concept rather than a specific instance. In many real situations, Portuguese prefers the article where English often does not.

Serviço militar means military service.

It usually refers to:

• service in the armed forces
• often compulsory military service, depending on context

On its own, the phrase does not have to mean only compulsory service, but in many contexts that is what people understand.

Because serviço is a masculine noun in Portuguese, so it takes:

• o serviço = the service

This is something you normally just have to learn with the noun. The ending -o often signals a masculine noun, and here that pattern fits.

Militar here is an adjective, and some Portuguese adjectives do not change form for masculine and feminine singular.

So you get:

• o serviço militar
• a carreira militar

In both cases, militar stays the same.

Terminou is the pretérito perfeito do indicativo, which is usually called the simple past or preterite in English explanations.

It comes from the verb terminar = to finish / to end.

So:

• terminar = to end
• terminou = ended / has ended

In this sentence, it refers to a completed event.

Terminou means ended and presents the event in a simple, direct way.

• O serviço militar terminou. = Military service ended.

Foi terminado is a passive structure meaning something more like was ended or was brought to an end, and it suggests an external agent more strongly.

Compare:

• O serviço militar terminou. = it ended
• O serviço militar foi terminado pelo governo. = it was ended by the government

So terminou is the natural choice unless you specifically want to emphasize who ended it.

Yes.

Terminar can work in two ways:

1. Intransitive: something ends

   • A reunião terminou. = The meeting ended.

2. Transitive: someone ends something

   • Eles terminaram a reunião. = They ended the meeting.

In O serviço militar terminou, it is being used intransitively.

The word order is the standard Portuguese pattern:

• Subject + verb
• O serviço militar
  • terminou

This is the most neutral and natural order.

You could change the order for style or emphasis in some contexts, but the basic sentence is perfectly normal as it stands.

In European Portuguese, the pronunciation is roughly:

• O often sounds like u
• serviço sounds roughly like sur-VEE-su
• militar sounds roughly like mee-lee-TAR
• terminou sounds roughly like tur-mee-NOH

A rough approximation for an English speaker could be:

• u sur-VEE-su mee-lee-TAR tur-mee-NOH

A few European Portuguese features to notice:

• unstressed vowels are often reduced
• final -o often sounds like u
• the ç in serviço is pronounced like s

The stressed syllables are:

• serVIço
• termiNOU

So:

• ser-VI-ço
• ter-mi-NOU

This is important because Portuguese stress is not always where an English speaker expects it.

The ç tells you that the c is pronounced like s before a, o, or u.

So:

• serviço = ser-vi-so

Without the cedilla, co, ca, and cu would normally have a hard k sound.

That is why Portuguese writes ç in words like:

• serviço
• coração
• começar

Yes, in many contexts you could say:

• O serviço militar acabou.

That also means The military service ended.

However, there can be a slight difference in feel:

• terminou = ended, finished
• acabou = ended, came to an end, was over

In this sentence, both are natural, but terminou can sound a little more neutral or formal.

By itself, it most naturally sounds like the military service as an institution, system, or specific known thing ended.

If you want to say that a particular person finished their military service, Portuguese would often make that clearer, for example:

• O serviço militar dele terminou. = His military service ended.
• Ele terminou o serviço militar. = He finished military service.

So context matters a lot.

Yes. Since serviço is masculine singular, the singular definite article is o.

Other possibilities include:

• um serviço militar = a military service
• os serviços militares = the military services
• serviço militar = military service, in a more general/article-less use

But in your sentence, o matches serviço naturally.

It is neutral and completely standard.

• not slang
• not especially formal
• not especially casual

It would work well in conversation, writing, or historical/political discussion.

You place não before the verb:

• O serviço militar não terminou. = The military service did not end.

This is the normal way to negate a sentence in Portuguese.

In Portuguese, you can often keep the same word order and just use intonation in speech or a question mark in writing:

• O serviço militar terminou? = Did the military service end?

You could also ask more explicitly:

• O serviço militar já terminou? = Has the military service ended already?

Portuguese does not usually need an auxiliary like did the way English does.