Segundo o extrato, o endividamento diminuiu.

Here segundo does not mean second. It means according to.

So:

• Segundo o extrato = According to the statement

This is a very common use in Portuguese, especially in formal or written language:

• Segundo o jornal... = According to the newspaper...
• Segundo a polícia... = According to the police...
• Segundo especialistas... = According to specialists...

The meaning second also exists, but the context makes the difference clear.

In Brazilian Portuguese, extrato often means a statement, especially a bank statement or account statement.

So in this sentence, o extrato most likely refers to some financial statement or account record.

Common examples:

• extrato bancário = bank statement
• extrato da conta = account statement

In other contexts, extrato can also mean extract, but in a sentence about debt, the financial meaning is the most likely one.

The comma separates an introductory phrase from the main clause.

Structure:

• Segundo o extrato, = introductory information
• o endividamento diminuiu. = main statement

This is very natural in Portuguese, just like in English:

• According to the statement, the indebtedness decreased.

You may also see a version without the comma in less careful writing, but with the comma it is clearer and more standard.

Endividamento means indebtedness, level of debt, or degree of being in debt.

It comes from the idea of becoming or being indebted. In financial contexts, it often refers not just to one debt, but to a person’s, company’s, or institution’s overall debt situation.

So there is a nuance:

• dívida = debt, a debt
• endividamento = indebtedness, debt level, debt burden

For example:

• A dívida aumentou. = The debt increased.
• O endividamento aumentou. = The indebtedness / overall debt level increased.

Portuguese uses definite articles much more often than English does.

So o endividamento literally has the, but in English we might translate it simply as:

• indebtedness decreased
• the indebtedness decreased
• debt levels decreased

All of these can work depending on context.

In Portuguese, using the article with abstract nouns is very common and often sounds more natural than leaving it out.

Diminuiu is the preterite form of the verb diminuir.

It is:

• 3rd person singular
• simple past
• used for a completed action in the past

So:

• o endividamento diminuiu = the indebtedness decreased

A few related forms:

• eu diminuí = I decreased
• ele/ela diminuiu = he/she/it decreased
• eles/elas diminuíram = they decreased

Here the subject is o endividamento, which is singular, so the verb is diminuiu.

Because here o endividamento is presented as the subject that decreased.

• O endividamento diminuiu. = The indebtedness decreased.

This is a normal active-style way to say that something went down.

If you said:

• O endividamento foi diminuído.

that would mean something more like:

• The indebtedness was reduced

That version suggests an external agent or action caused the reduction more explicitly. It is grammatical, but it sounds more passive and less natural in many contexts.

Yes, you could, but the meaning is slightly different.

• A dívida diminuiu. = The debt decreased.
• O endividamento diminuiu. = The indebtedness / overall debt level decreased.

Dívida is more direct and concrete.
Endividamento is a bit more abstract and often sounds more formal, technical, or financial.

So if the source is a financial statement, endividamento fits very well.

Yes. A very natural alternative is:

• O endividamento diminuiu, segundo o extrato.

This means the same thing: According to the statement, the indebtedness decreased.

The difference is mostly one of emphasis:

• Segundo o extrato, o endividamento diminuiu.
  Starts with the source of the information.
• O endividamento diminuiu, segundo o extrato.
  Starts with the main fact.

Both are correct.

In Brazilian Portuguese, diminuiu is pronounced approximately like jee-mee-noo-EE-oo, though that English-style approximation is not perfect.

A more useful breakdown is:

• di-mi-nu-iu

The important part is the ending -iu, which is pronounced clearly. The stress falls on the iu part:

• di-mi-nu-IU

Also:

• diminuir = the infinitive
• diminuiu = past tense, decreased

Learners often want to compress the ending too much, but in Portuguese it is better to pronounce the vowel sequence clearly.

Yes, it sounds fairly neutral-to-formal, especially because of words like extrato and endividamento.

It would fit well in:

• financial reports
• news articles
• business discussions
• formal written Portuguese

In casual everyday conversation, people might choose simpler wording depending on context, but this sentence is completely natural Brazilian Portuguese.