La declaración conjunta cambia el resultado de la declaración.

Because the two uses refer to related but slightly different things:

• la declaración conjunta = the joint tax return / joint filing
• el resultado de la declaración = the result of the tax return / the filing outcome

So the sentence is saying that filing jointly changes the result of the tax return.

In English, repeating declaration might sound a bit awkward, but in Spanish this kind of repetition is often perfectly normal. In a real conversation, a native speaker might also rephrase it to avoid repetition, depending on context.

In Spain, declaración conjunta usually refers to a joint tax return, especially in the context of income tax.

• declaración can mean statement, declaration, or tax return
• conjunta means joint

So in this context, it is best understood as joint filing or a joint tax return, not just any kind of declaration.

Because conjunta is an adjective that must agree with declaración, which is a feminine singular noun.

• la declaración → feminine singular
• therefore: conjunta → feminine singular

Compare:

• el documento conjunto = the joint document
• la declaración conjunta = the joint declaration/tax return

La is the definite article meaning the, and it goes with declaración because declaración is feminine singular.

So:

• la declaración = the declaration / the tax return

Spanish uses definite articles more often than English does, so even where English might sometimes omit the, Spanish often keeps it.

Because resultado is a masculine singular noun, so it takes el.

• el resultado = the result

This gives us:

• La declaración conjunta = feminine singular
• el resultado = masculine singular

The articles change depending on the gender of the noun they go with.

Cambia is the third person singular present tense of cambiar = to change.

So here it means:

• changes

Why third person singular?

Because the subject is La declaración conjunta, which is singular:

• La declaración conjunta cambia... = The joint return changes...

Because resultado is the direct object of cambiar, not an indirect object or a person.

• cambiar algo = to change something

So:

• cambia el resultado = changes the result

You would not use a here. The personal a is mainly used with specific people, and resultado is a thing, not a person.

This means of the declaration / of the tax return.

It is made of:

• de = of
• la declaración = the declaration

When you put them together:

• de la declaración = of the declaration

This phrase tells us which result we are talking about: the result of the tax return.

Because in Spanish, only these two common contractions happen:

• de + el = del
• a + el = al

But de + la does not contract.

So:

• del resultado = from/of the result
• de la declaración = of the declaration

That is why de la declaración stays exactly as it is.

Yes, it is completely normal.

The basic structure is:

• Subject: La declaración conjunta
• Verb: cambia
• Object: el resultado de la declaración

So it follows a standard Spanish word order similar to English:

• The joint return changes the result of the return

Spanish can be flexible with word order, but this version is straightforward and natural.

Yes. On its own, declaración can mean several things, such as:

• declaration
• statement
• announcement
• tax return

The exact meaning depends on context. In this sentence, because of declaración conjunta, the tax meaning is the most likely one.

Yes. Since declaración appears twice, a native speaker might choose a different wording if the context is clear. For example:

• La declaración conjunta cambia el resultado final.
• Hacer la declaración conjunta cambia el resultado.
• La tributación conjunta cambia el resultado de la declaración.

But the original sentence is still grammatically correct.

A careful pronunciation would be roughly:

la de-kla-ra-THYON kon-HOON-ta KAM-bya el re-sul-TA-do de la de-kla-ra-THYON

A few Spain-specific notes:

• In most of Spain, ci in declaración sounds like thy in a soft English approximation.
• j in conjunta is a strong throaty sound, not like English j.
• The stress falls on:
  • declaración
  • conjunta
  • cambia
  • resultado

If the learner is using Latin American pronunciation, the sentence is still the same in spelling, just pronounced differently.