La reducción fiscal cambia la base liquidable.

Because both nouns are feminine singular:

• la reducción
• la base

In Spanish, nouns usually need an article more often than in English, especially in full statements like this one. So La reducción fiscal cambia la base liquidable sounds natural and complete, while leaving out the articles would sound unnatural in standard Spanish.

Reducción is a noun ending in -ción, and nouns with -ción are almost always feminine in Spanish.

Examples:

• la reducción
• la situación
• la administración

So the adjective/article agreement must match:

• la reducción fiscal

Fiscal is an adjective with the same form for masculine and feminine singular nouns.

So you get:

• el sistema fiscal
• la reducción fiscal

Many Spanish adjectives ending in -al, -ar, -il, -or (sometimes with agreement only in number), etc., do not change for masculine/feminine in the singular.

Because the subject is la reducción fiscal, which is third-person singular.

The infinitive is cambiar. In the present tense:

• yo cambio
• tú cambias
• él/ella cambia

So:

• La reducción fiscal cambia...

The verb agrees with the subject, not with the object.

The subject is La reducción fiscal because it is the thing doing the action of cambiar.

Structure:

• La reducción fiscal = subject
• cambia = verb
• la base liquidable = direct object

A good test in English is: what is changing something?
Answer: the tax reduction.

That is the normal word order in Spanish for a simple statement:

subject + verb + object

So:

• La reducción fiscal
  • cambia
    • la base liquidable

Spanish word order is more flexible than English, but this is the most neutral and standard order here.

In Spanish, most descriptive adjectives normally come after the noun.

So:

• base liquidable not usually
• liquidable base

Also, base liquidable is a fixed technical expression in tax language, so the adjective naturally follows the noun.

It is an adjective formed with -able, similar to English -able in words like payable or taxable.

Here it comes from the idea of something being able to be calculated/settled for tax purposes. In everyday Spanish, liquidable is not especially common, but in legal and tax language it is normal.

In this sentence, it is part of a technical term: base liquidable.

It is mainly tax jargon. A learner might understand the words separately, but together they form a technical tax term.

In Spanish tax language, terms like these are often fixed expressions, and they may not sound natural outside that context.

A related term you may also see is base imponible, which is another technical tax term. It is not exactly the same thing, so in tax contexts it is best to learn these as set phrases.

Sometimes, yes, but not always with exactly the same nuance.

• reducción fiscal sounds more formal and technical
• rebaja fiscal can also mean a tax cut, but often sounds a bit more general or political/journalistic

In a formal tax or legal sentence, reducción fiscal fits better.

Not in normal standard Spanish. Spanish generally uses articles in sentences like this.

So the natural form is:

• La reducción fiscal cambia la base liquidable.

Dropping both articles would sound like a headline, notes, or very unnatural learner Spanish.

Because la base liquidable is a direct object, and with things, Spanish usually does not use a preposition before the direct object.

Compare:

• La reducción fiscal cambia la base liquidable.
• El impuesto afecta la renta.

You only get the personal a with specific human direct objects:

• Veo a María.

Since la base liquidable is not a person, there is no a.

It is pronounced roughly reh-dook-THYON in most of Spain, or reh-dook-SYON in Latin America and some parts of Spain.

A few useful points:

• the written accent in reducción shows the stress falls on the last syllable
• ción is a very common ending
• in Spain, ci is often pronounced with a th sound

So in Spain:

• reducción ≈ reh-dook-THYON

Because without the accent mark, Spanish stress rules would make you pronounce it differently.

Spanish words ending in -n, -s, or a vowel are normally stressed on the second-to-last syllable. But reducción is stressed on the last syllable, so it needs a written accent:

• reducción

This is very common with nouns ending in -ción:

• nación
• situación
• administración