A investigação começa hoje.

Because a is the feminine singular definite article, meaning the.

So A investigação = The investigation.

Portuguese uses definite articles very naturally in places where English also uses the, and sometimes even where English would omit it. In this sentence, it sounds like a specific investigation that both speaker and listener can identify.

If you said Uma investigação começa hoje, that would mean An investigation starts today, which sounds less specific.

Investigação is a feminine noun, so it takes a, not o.

A very useful pattern is that nouns ending in -ção are almost always feminine in Portuguese:

• a investigação
• a situação
• a informação
• a operação

So once you see -ção, feminine gender is usually a very safe guess.

Começa is the 3rd person singular present tense form of the verb começar, which means to begin or to start.

Here is the relevant part of the conjugation:

• eu começo = I start
• você / ele / ela começa = you / he / she starts
• a investigação começa = the investigation starts

Since a investigação is singular, the verb is also singular: começa.

The ç is called cedilha. It makes the c sound like s before a, o, or u.

So:

• ca normally sounds like k
• ça sounds like sa

That means começa is pronounced with an s sound in that syllable, not a k sound.

This spelling happens because the infinitive is começar. The cedilla keeps the soft s sound in forms where c comes before a:

• começar
• começa
• começamos

But before e or i, regular c already has the soft sound, so no cedilla is needed:

• comecei
• comece

It is grammatically present tense, but in this sentence it often refers to something that starts today, including a scheduled event beginning later the same day.

Portuguese, like English, often uses the present tense for planned or scheduled actions:

• A investigação começa hoje.
• O filme começa às oito.
• As aulas começam amanhã.

So even though the verb is in the present, the idea can be very close to is starting today or starts today.

A rough Brazilian Portuguese pronunciation guide would be:

• A = ah
• investigação ≈ in-ves-chee-gah-SOWN
• começa ≈ ko-MEH-sa
• hoje ≈ HO-zhee or OH-zhee, depending on accent

A few key pronunciation points:

• ç = s sound
• j in hoje sounds like the s in measure or the zh sound
• ão is nasal and does not sound exactly like anything in English
• the stress in investigação falls on the last syllable: -ção
• the stress in começa falls on me

Because that is a very normal and neutral place for a time word in Portuguese.

Hoje means today, and adverbs of time are fairly flexible. All of these are possible:

• A investigação começa hoje.
• Hoje, a investigação começa.
• Começa hoje a investigação.

The version in your sentence sounds very natural and straightforward.

Putting hoje at the beginning gives it a little more emphasis:

• Hoje, a investigação começa. = Today, the investigation starts.

Because hoje is an adverb, not a noun.

It simply means today, so it does not take an article in this use.

Compare:

• hoje = today
• amanhã = tomorrow
• ontem = yesterday

These words normally appear without o / a.

Yes. That is also very common in Brazilian Portuguese.

• A investigação começa hoje. = more direct, simple, neutral
• A investigação vai começar hoje. = explicitly future, often a little more conversational

Both are correct. In everyday Brazilian Portuguese, vai começar is very common when speakers want to make the future meaning extra clear.

Not naturally here, unless the context already makes the subject obvious.

Portuguese often drops subject pronouns like eu, ele, ela, but dropping a full noun phrase like a investigação is different. If you are introducing the idea, you usually keep it:

• A investigação começa hoje.

If the context already established what is starting, then you might just say:

• Começa hoje.

But by itself, that would mean It starts today or Starts today, and the listener would need context to know what it is.

Usually yes, but it depends on context.

Investigação works well for:

• police investigation
• formal inquiry
• investigation of a case or event

But English research is often pesquisa in Brazilian Portuguese, not investigação:

• pesquisa científica = scientific research
• investigação policial = police investigation

So investigação and pesquisa are related ideas, but they are not always interchangeable.