O roubo aconteceu ontem.

O is the masculine singular definite article, equivalent to the.

So o roubo means the robbery. In this sentence, it usually suggests a specific robbery, or one that is already known in the context.

Portuguese uses articles very often, sometimes more often than English does, so seeing o here is completely normal.

Yes, you could.

• O roubo aconteceu ontem = the robbery happened yesterday
• Um roubo aconteceu ontem = a robbery happened yesterday

The difference is specificity:

• o roubo points to a specific robbery
• um roubo introduces it as one robbery among others, or as new information

So both are grammatical, but they do not mean exactly the same thing.

Here, roubo is a noun meaning robbery or theft, depending on context.

It comes from the verb roubar, meaning to rob or to steal.

A very common beginner confusion is with roubou, which is a verb form:

• roubo = robbery / theft
• roubou = robbed / stole

So in this sentence, o roubo is the subject of the sentence.

Aconteceu is the preterite form of acontecer.

More specifically, it is:

• 3rd person singular
• preterite tense
• from the verb acontecer = to happen

So it means happened.

Because the subject is o roubo, which is singular, the verb is also singular: aconteceu.

Because the subject is singular.

• o roubo = singular
• so the verb must also be singular: aconteceu

If the subject were plural, then you would use aconteceram:

• Os roubos aconteceram ontem = The robberies happened yesterday

This is normal subject-verb agreement.

Yes. O roubo ocorreu ontem is also correct.

Both aconteceu and ocorreu can mean happened/occurred.

The difference is mostly style:

• aconteceu sounds more common and everyday
• ocorreu sounds a little more formal or report-like

So in normal conversation, aconteceu is often the more natural choice.

Putting ontem at the end is very natural in Portuguese.

This sentence follows a common pattern:

• subject + verb + time expression

So:

• O roubo aconteceu ontem

That said, ontem can also appear in other positions for emphasis or style:

• Ontem, o roubo aconteceu
• O roubo ontem aconteceu — possible, but less natural in most situations

The original sentence sounds very standard and neutral.

Yes:

• Ontem, o roubo aconteceu.

This is grammatical, and it puts more focus on yesterday.

In everyday Portuguese, both are possible:

• O roubo aconteceu ontem.
• Ontem, o roubo aconteceu.

The first one is usually the more neutral version.

A rough Brazilian pronunciation is:

• O → often sounds like oo
• roubo → roughly HOH-boo or HOH-bo
• aconteceu → roughly ah-kon-teh-SEH-oo
• ontem → roughly ON-teng with nasal sound

A more IPA-style version is approximately:

• o roubo aconteceu ontem
• [u ˈʁowbu akõteˈsew ˈõtẽj̃]

A few helpful notes:

• the r at the start of roubo is usually pronounced like a breathy h in many Brazilian accents
• ou in roubo sounds like a diphthong similar to oh
• ontem has nasal vowels, which English does not really have

These words are related, but they are not identical.

• roubo: robbery; in legal/useful everyday terms, it often involves taking something through force or threat
• furto: theft; usually taking something without force or direct confrontation
• assalto: mugging / hold-up / armed robbery; often emphasizes the attack or threatening situation

So:

• roubo is the word used in your sentence
• furto would not always fit the same situation
• assalto focuses more on the violent or threatening act itself

In casual English, people may sometimes translate more than one of these as robbery or theft, but in Portuguese they are distinct words.

No.

Ontem means yesterday, and it is used by itself:

• aconteceu ontem
• cheguei ontem
• vi ele ontem

This works a lot like English, where you also just say yesterday, not on yesterday.

Yes, absolutely.

Acontecer is a very general verb meaning to happen, and it can be used for:

• good things
• bad things
• neutral things

Examples:

• Isso aconteceu ontem. = That happened yesterday.
• Um acidente aconteceu na estrada. = An accident happened on the road.
• O roubo aconteceu ontem. = your sentence

So there is nothing unusual about using aconteceu with roubo.