El pronto pago cambia el importe de la sanción.

Pronto pago is a fixed expression meaning early payment, prompt payment, or payment within the reduced deadline.

In Spain, it often appears in official or administrative language, especially with fines or penalties. It usually implies that if you pay quickly, the amount may be reduced.

So in this sentence, el pronto pago is not just paying soon in a general sense; it is more like the formal idea of paying early under the rules of the notice.

Because el pronto pago is being used as a noun phrase, specifically the subject of the sentence.

• pronto pago = early payment / prompt payment
• el pronto pago = the early payment / early payment as a concept in this case

Spanish often uses the definite article with abstract or formal concepts where English may or may not use one.

Here, el pronto pago cambia... literally means the early payment changes..., but in natural English we might simply say early payment changes....

It is best understood as a set expression.

Even though:

• pronto usually means soon or prompt
• pago means payment

together pronto pago works as a fixed administrative expression meaning prompt/early payment.

That is why learners should memorize it as a chunk, especially for official Spanish.

Because the subject is singular: el pronto pago.

So the verb must also be singular:

• el pronto pago cambia
• not el pronto pago cambian

This is simple subject-verb agreement.

Because the sentence is stating a general fact or rule.

Spanish often uses the present indicative for this kind of factual statement:

• El pronto pago cambia el importe de la sanción.
• Early payment changes the amount of the penalty/fine.

It is not talking about a possible or doubtful action, so there is no need for the subjunctive.

Importe means amount, especially a sum of money to be paid.

It is very common in formal, financial, and administrative Spanish.

So:

• el importe de la sanción = the amount of the penalty/fine

Compared with other words:

• cantidad = amount/quantity, more general
• importe = monetary amount, especially in official contexts
• monto = also amount, but more common in much of Latin America than in Spain

Because el importe is being specified: it is the amount of the penalty/sanction.

This is a very common Spanish structure:

• el importe de la factura = the amount of the invoice
• el importe de la multa = the amount of the fine
• el importe de la sanción = the amount of the penalty

The article la is there because it refers to a specific sanction/penalty in context.

They are related, but not always identical.

• multa = a fine, specifically a money penalty
• sanción = a broader penalty or sanction

So sanción can be more formal and slightly wider in meaning. In many traffic or administrative contexts, it may still refer to a financial penalty, but multa is more specific.

That means:

• el importe de la multa = the amount of the fine
• el importe de la sanción = the amount of the penalty/sanction

Yes. A very natural alternative would be:

El importe de la sanción cambia con el pronto pago.

This means essentially the same thing: The amount of the penalty changes with early payment.

The original sentence puts the focus on el pronto pago as the thing causing the change:

• El pronto pago cambia...

The alternative puts the focus on el importe de la sanción:

• El importe de la sanción cambia...

Both are grammatically correct.

Not in this meaning.

For this formal expression, the usual phrase is pronto pago.

• pronto pago = prompt/early payment
• pago pronto sounds unnatural here

If you want to express the general idea to pay soon/quickly, you would normally use a verb phrase such as:

• pagar pronto

But that is not quite the same as the fixed administrative term pronto pago.

Because cambiar can take a direct object directly.

Here:

• cambia = changes
• el importe = the direct object

So:

• El pronto pago cambia el importe de la sanción.

This is normal Spanish syntax.

Compare:

• Eso cambia el resultado.
• La oferta cambia el precio.

No preposition is needed before the thing being changed.

It is grammatical and clear, but it sounds somewhat formal and administrative.

You are most likely to see language like this in:

• official notices
• traffic fine information
• payment conditions
• legal or bureaucratic texts

In everyday conversation, people might say something simpler, such as:

• Si pagas pronto, pagas menos.
• El pago anticipado reduce la multa.

But the original sentence is perfectly natural in official Spanish.