Quero retificar e regularizar a declaração do Imposto de Renda.

Because Portuguese often leaves out the subject pronoun when the verb already makes the subject clear.

• Quero = I want
• The -o ending shows it is I

So Quero retificar... is a perfectly natural way to say I want to amend...
You can say Eu quero if you want emphasis, contrast, or extra clarity, but it is not necessary here.

After querer (to want), Portuguese normally uses another verb in the infinitive.

So:

• Quero retificar = I want to amend
• Quero regularizar = I want to regularize

When two actions are connected to the same quero, you can simply join them with e:

• Quero retificar e regularizar...

This is just like English I want to amend and regularize...

They are related, but not the same.

• retificar = to correct, amend, or formally fix information in a document
• regularizar = to make something legally or administratively regular, compliant, or in order

In a tax context:

• retificar a declaração often means amend/correct the tax return
• regularizar a declaração suggests bringing the situation into compliance or sorting out any pending issue

So the sentence is not just repeating the same idea twice. It suggests both correcting the declaration and making its status proper/regular.

Yes, in this context it usually refers to the income tax return.

Literally, it is something like:

• declaração = declaration/return
• do Imposto de Renda = of income tax

In real Brazilian usage, declaração do Imposto de Renda is a normal way to refer to the document you file for income tax.

Portuguese uses definite articles more often than English.

Here, a is the feminine singular definite article, meaning the:

• a declaração = the declaration / the return

It sounds natural because you are talking about a specific declaration, usually your own or one already known from context.

English often says just my tax return or the tax return, but Portuguese very commonly includes the article.

do is a contraction of:

• de = of
• o = the

So:

• do = of the

That means:

• declaração do Imposto de Renda = declaration of the income tax

This kind of contraction is extremely common in Portuguese:

• de + o = do
• de + a = da
• em + o = no
• em + a = na

Because it is often treated as the name of a specific official tax, especially in formal or administrative writing.

You will also see imposto de renda in lowercase, especially in less formal contexts. Both appear in real Brazilian Portuguese.

A simple way to think about it:

• Imposto de Renda with capitals = more official/document-style
• imposto de renda lowercase = more general or less formal

Yes, but there is a nuance.

• corrigir = to correct
• retificar = to formally amend/correct, especially in official or legal contexts

In everyday speech, many people might say:

• Quero corrigir minha declaração...

But in tax or bureaucratic language, retificar is more precise and more natural. It sounds like the official act of filing a correction or amendment.

So corrigir is understandable, but retificar fits this context better.

Because Portuguese can leave possession implied when the context already makes it obvious.

So both are possible:

• Quero retificar e regularizar a declaração do Imposto de Renda.
• Quero retificar e regularizar minha declaração do Imposto de Renda.

The version with minha is more explicit.
The version without minha can sound a little more neutral or administrative.

It is natural, but it does sound somewhat formal or administrative.

That makes sense, because the topic is tax paperwork. In Brazil, when talking about documents, forms, legal issues, or government procedures, this kind of wording is very common.

A more everyday version might be:

• Quero corrigir e regularizar minha declaração de imposto de renda.
• Preciso retificar minha declaração do Imposto de Renda.

So the original sentence is natural, especially in a tax, legal, or office context.

Both versions can appear, but they are not felt in exactly the same way.

• declaração do Imposto de Renda sounds like the income tax return, referring to the specific official filing connected to that tax
• declaração de imposto de renda can sound a bit more general, like income tax declaration as a type of document

In practice, Brazilians use both, but declaração do Imposto de Renda is very common when referring to the specific annual return in a more formal or official way.