Declarei o sinistro ontem.

Declarei is the pretérito perfeito simples in European Portuguese. It is the most normal tense for a completed action at a specific time in the past.

So Declarei o sinistro ontem means the action was done and finished yesterday.

• declarei = I declared / I reported
• declarava would suggest a repeated, ongoing, or background past action
• tenho declarado in Portuguese does not work like English I have declared in many cases; in European Portuguese it often suggests something repeated over time, not simply one finished action yesterday

Because the sentence includes ontem, the simple past declarei is the natural choice.

Declarei is:

• 1st person singular = I
• from the verb declarar
• in the pretérito perfeito simples

A quick mini-pattern:

• eu declarei = I declared
• tu declaraste = you declared
• ele/ela declarou = he/she declared
• nós declarámos = we declared
• vós declarastes = you (plural, old/literary) declared
• eles/elas declararam = they declared

In the sentence, the subject eu is omitted because Portuguese often drops subject pronouns when the verb ending already makes the subject clear.

Portuguese is a pro-drop language, which means subject pronouns are often left out when the verb already shows who is doing the action.

So:

• Declarei o sinistro ontem = perfectly natural
• Eu declarei o sinistro ontem = also correct, but more emphatic

You would add eu if you want contrast or emphasis, for example:

• Eu declarei o sinistro ontem, não foi o meu irmão.

No. In this sentence, sinistro is an insurance term.

It usually means something like:

• an insurance claim
• an insured incident
• a loss event
• an accident/damage event reported to an insurer

So this is a case where the Portuguese word looks like English sinister, but the meaning here is quite different.

In insurance language, declarar o sinistro means to report the claim/incident to the insurance company.

Because sinistro is a masculine singular noun, and here it is being used as a specific, identifiable thing: the claim / the incident.

So:

• o sinistro = the claim / the incident
• um sinistro = a claim / an incident

In many real-life situations, the speaker and listener already know which incident is meant, so o sinistro is very natural.

Possibly, but Declarei o sinistro is a common insurance-style expression.

Some related options are:

• Declarei o sinistro — standard and natural in insurance contexts
• Participei o sinistro — also used in insurance/legal contexts, though usage can vary
• Reportei o sinistro — understandable, but may sound less idiomatic depending on context
• Comuniquei o sinistro à seguradora — I informed the insurer about the claim
• Fiz a participação do sinistro — I filed the claim/report

So declarar is a good verb here because it matches formal insurance language well.

Yes, ontem can move.

The sentence Declarei o sinistro ontem is a very normal neutral word order:

• verb + object + time expression

You can also say:

• Ontem declarei o sinistro.
• Ontem, declarei o sinistro.

Putting ontem first gives it a bit more prominence. Both versions are correct.

The end position is especially common when the speaker is simply adding time information without special emphasis.

In European Portuguese, declarei is approximately:

• deh-clah-RAY

A bit more carefully:

• the stress is on the last syllable: -rei
• the r in declarar/declarei is usually the single-tap r sound in the middle of a word
• unstressed vowels in European Portuguese are often reduced, so it may sound less open and less fully pronounced than an English speaker expects

A rough approximation is:

• d(ɨ)-cla-REI

The final -ei sounds like ay in say, though the exact quality depends on accent and speed.

In European Portuguese, ontem is roughly:

• ON-tãj
• or more broadly for learners, ON-teng with a nasal ending, though that is only an approximation

Important points:

• the first syllable is stressed: ON
• the final sound is nasal
• the m at the end does not sound like a full English m; it mainly nasalizes the vowel before it

European Portuguese nasal vowels can be tricky at first, so it is normal if your pronunciation is only approximate in the beginning.

It is mostly neutral to formal, mainly because of the noun sinistro and the expression declarar o sinistro, which belong strongly to insurance vocabulary.

In everyday casual conversation, someone might choose more everyday wording depending on context, for example:

• Ontem participei o acidente à seguradora.
• Ontem fiz a participação do acidente.
• Ontem avisei a seguradora.

But if you are talking specifically about an insurance claim, Declarei o sinistro ontem sounds appropriate and natural.

No. In insurance language, sinistro can refer to different kinds of insured incidents, depending on the policy, such as:

• a car accident
• water damage at home
• theft
• fire
• other insured losses

So it is broader than just accident. The exact meaning depends on the insurance context.

Yes. You can add that information easily, for example:

• Declarei o sinistro à seguradora ontem.
  = I reported the claim to the insurer yesterday.

• Declarei o sinistro ontem à seguradora.

• Ontem declarei o sinistro à companhia de seguros.

All of these are grammatical. The choice depends on which information you want to highlight.

The infinitive is declarar, and yes, it is a regular -ar verb.

That is helpful because once you know regular -ar past forms, many verbs follow the same pattern:

• falar → falei
• estudar → estudei
• declarar → declarei

So declarei follows a very common and predictable pattern.