A declaração pré-preenchida mostra a fonte pagadora.

Because declaração is a feminine noun in Portuguese, so it takes the feminine singular article a.

A few related points:

• a = the for feminine singular nouns
• o = the for masculine singular nouns
• nouns ending in -ção are very often feminine:
  
  • a declaração
  • a informação
  • a situação

So a declaração is simply the correct gender/article combination.

Pré-preenchida means pre-filled or already filled in beforehand.

It is made of:

• pré- = pre-
• preenchida = filled in

The hyphen is used because the prefix pré- keeps its stress and accent in this form. In standard spelling, forms with pré- like this are commonly hyphenated:

• pré-pago
• pré-natal
• pré-preenchido / pré-preenchida

So declaração pré-preenchida means pre-filled tax return / pre-filled declaration.

Because adjectives in Portuguese usually agree with the noun they describe in gender and number.

Here:

• declaração is feminine singular
• so the adjective must also be feminine singular: pré-preenchida

Compare:

• o formulário pré-preenchido = the pre-filled form
• a declaração pré-preenchida = the pre-filled declaration

So pré-preenchida matches declaração.

Literally, it means paying source, but in Brazilian Portuguese it is a technical tax/financial term.

In practice, fonte pagadora usually refers to the person, company, employer, bank, or institution that paid the income being reported.

Depending on context, English translations may include:

• payer
• paying source
• income source
• entity that paid the income

So this is not just any source in a general sense; it is specifically the source that made the payment.

Because pagadora agrees with fonte, and fonte is feminine in Portuguese.

So:

• a fonte pagadora
• not a fonte pagador

This is the same agreement pattern as:

• a empresa pagadora
• a instituição pagadora

Even if the actual payer is a company, bank, or employer, the adjective follows the grammatical gender of fonte, not the real-world sex of a person.

Mostra is the third person singular present indicative form of the verb mostrar (to show).

It is used because the subject is singular:

• A declaração pré-preenchida = singular subject
• therefore: mostra

Conjugation:

• eu mostro
• você/ele/ela mostra
• nós mostramos
• eles/elas mostram

So the sentence structure is:

• A declaração pré-preenchida = subject
• mostra = verb
• a fonte pagadora = object

The present tense in Portuguese is often used for general facts, instructions, interface text, and descriptions of what something does.

So mostra here means something like:

• shows
• displays
• will show in a general procedural sense, depending on context

This is very natural in Portuguese, especially in official instructions, websites, apps, and forms.

For example:

• O sistema mostra os dados automaticamente.
• A página exibe o resultado.

It describes a normal function or feature.

Yes, very much so.

In Brazilian tax language, declaração pré-preenchida is a common official expression, especially in the context of income tax filing. It refers to a return/declaration that already comes with data filled in from government or institutional records.

So while you can understand it word by word, it is also a set phrase commonly used in real Brazilian administrative language.

Yes, but the original order is the most natural and neutral one.

Standard order:

• A declaração pré-preenchida mostra a fonte pagadora.

You could rearrange it in special contexts for emphasis, but it would sound less neutral or more marked. For example:

• A fonte pagadora é mostrada pela declaração pré-preenchida.
  This is grammatical, but more passive and less natural in everyday instructional language.

So for normal use, the original sentence is the best choice.

Either can work, depending on context.

• shows is the most direct general translation
• displays may sound more natural if this is about a website, app, or tax software interface

In Portuguese, mostrar is broad and flexible, so it can cover both ideas. If this sentence appears in a digital or bureaucratic context, English might naturally use shows, displays, or even lists, depending on what is being referred to.

A rough English-friendly pronunciation is:

• declaração ≈ deh-clah-rah-SOWN
• pré ≈ preh
• preenchida ≈ preh-en-SHEE-dah

A few pronunciation notes:

• ção sounds like sown with a nasal ending
• pré has a clear open eh sound
• ch in preenchida sounds like sh
• the stress is on:
  • declaraÇÃO
  • PRÉ
  • preenCHIda

This is only approximate, but it helps an English speaker get close.

Not in normal standard Portuguese.

Portuguese uses articles much more often than English does, especially in full, natural sentences. So:

• A declaração pré-preenchida mostra a fonte pagadora. = natural
• Declaração pré-preenchida mostra fonte pagadora. = sounds unnatural or headline-like

You may sometimes see article omission in titles, menu labels, or very compressed notes, but in a regular sentence the articles should stay.