Uma vez que a pendência fiscal continua, preciso retificar.

Here uma vez que introduces the reason for the action in the main clause. It works like:

• since
• because
• given that

So in this sentence, it is not talking about time first; it is giving a justification: because the tax issue is still unresolved, I need to correct/amend something.

Yes. Depending on context, uma vez que can sometimes feel closer to once or once it is the case that.

For example:

• Uma vez que ele chegue, começamos.
  = Once he arrives, we start.

But in your sentence, the most natural reading is causal, not temporal:

• Uma vez que a pendência fiscal continua...
  = Since/Given that the tax issue continues...

So this is a good expression to learn as a formal way to say since/because.

The comma separates the subordinate clause from the main clause.

Structure:

• Uma vez que a pendência fiscal continua = dependent/reason clause
• preciso retificar = main clause

In English, we also often use a comma when a reason or time clause comes first:

• Since the issue remains unresolved, I need to amend it.

If the order were reversed, the comma would often be optional or less natural:

• Preciso retificar, uma vez que a pendência fiscal continua.

Portuguese often uses the definite article where English might not.

Here a pendência fiscal suggests a specific pending tax issue, or a tax matter already known in the context.

So:

• a pendência fiscal = the tax issue / the pending tax matter

Without the article, pendência fiscal would sound more generic or dictionary-like, not as natural in this full sentence.

It is a fairly formal or administrative expression.

• pendência = something pending, unresolved, outstanding
• fiscal = tax-related / fiscal

So pendência fiscal usually means:

• an unresolved tax issue
• a pending tax matter
• an outstanding tax irregularity or obligation

It is the kind of phrase you often see in bureaucracy, accounting, legal, or government contexts.

Because the subject is singular:

• a pendência fiscal = singular
• therefore the verb must be singular: continua

If the subject were plural, then you would use continuam:

• As pendências fiscais continuam.
  = The tax issues continue.

Portuguese often drops subject pronouns when the verb form already makes the subject clear.

• preciso = I need
• precisa = you/he/she need(s) (depending on the pronoun)
• precisamos = we need

Since preciso already clearly signals I, eu is usually unnecessary.

So:

• Eu preciso retificar = correct, but more explicit
• Preciso retificar = also correct, and very natural

In this sentence, it clearly means I need, because it is followed by an infinitive verb:

• preciso retificar = I need to amend/correct

As an adjective, preciso can mean precise or accurate:

• O valor é preciso.
  = The value is precise/accurate.

But that is a different structure. When preciso is followed by another verb in the infinitive, it is almost certainly the verb precisar:

• preciso fazer
• preciso corrigir
• preciso retificar

Retificar is a formal verb meaning something like:

• to rectify
• to correct
• to amend
• to make a correction in an official record or document

It is more formal and administrative than everyday corrigir.

For example:

• corrigir = correct a mistake in general
• retificar = correct or amend information formally, especially in documents, filings, declarations, records, etc.

So this sentence sounds like something from official, legal, accounting, or tax language.

Because retificar sounds more appropriate for formal paperwork or official information.

Compare:

• Preciso corrigir o exercício.
  = I need to correct the exercise.
• Preciso retificar a declaração.
  = I need to amend/correct the declaration.

In tax, legal, and bureaucratic contexts, retificar is often the preferred word because it suggests an official correction to a filed document or registered information.

They are close, but not identical in tone.

• preciso retificar = I need to amend/correct
• tenho que retificar = I have to amend/correct

In many contexts, both work. But:

• preciso can sound a bit more neutral or internal: there is a need
• tenho que can sound a bit more forceful: I have to / I’m obliged to

In formal writing, preciso retificar often sounds smoother and slightly more polished.

Yes, it sounds quite formal, especially because of:

• uma vez que
• pendência fiscal
• retificar

A more everyday version might use simpler wording, such as:

• Como o problema fiscal continua, preciso corrigir isso.

But the original sentence is very natural in administrative, legal, tax, or professional contexts.