El formulario aduanero contiene la declaración aduanera.

It means customs-related. It comes from aduana, which means customs or customs office.

In Spanish, adjectives change to match the noun they describe:

• formulario aduanero: formulario is masculine singular, so aduanero
• declaración aduanera: declaración is feminine singular, so aduanera

So the change is just normal adjective agreement.

Because Spanish nouns have grammatical gender.

• formulario is masculine, so it takes el
• declaración is feminine, so it takes la

A very helpful pattern: nouns ending in -ción are usually feminine:

• la declaración
• la información
• la conversación

So la declaración follows a very common rule.

Because in Spanish, adjectives usually come after the noun, especially when they classify the noun or describe its type.

So:

• formulario aduanero
• declaración aduanera

is the normal word order.

Putting the adjective before the noun here would sound unusual and not like standard official Spanish.

Contiene comes from the infinitive contener, which means to contain.

Here it is in the third person singular present tense:

• yo contengo
• tú contienes
• él/ella/usted contiene

It matches the subject el formulario aduanero, which is singular.

Also, contener is related to tener, so its forms are a little irregular.

Yes. In this kind of official or administrative sentence, contiene can often be understood as contains or includes.

So the idea is not necessarily that one physical object is inside another. It can also mean that the form includes a customs declaration section or information.

In everyday English, includes may sound more natural, but contiene is perfectly normal Spanish.

Yes, especially for an official document that you fill out.

Formulario is very common for:

• application forms
• registration forms
• customs forms
• tax forms

By contrast, forma usually means shape, form, or manner, and is not usually the best choice for a bureaucratic document.

So formulario aduanero is a natural phrase.

Yes. Both are understandable and natural.

• declaración aduanera sounds a bit more formal or technical
• declaración de aduana is also common and clear

Spanish often has both patterns:

• adjective: declaración aduanera
• de + noun: declaración de aduana

In official language, the adjective version is very common.

Because the stress falls on the last syllable: de-cla-ra-CIÓN.

Normally, words ending in a vowel, n, or s are stressed on the second-to-last syllable. Since declaración ends in n but is stressed on the last syllable instead, it needs a written accent mark.

In most of Latin America, -ción sounds roughly like syon.

Usually not in a normal full sentence.

Spanish uses definite articles more often than English does, so:

• El formulario aduanero contiene la declaración aduanera is natural
• leaving out the articles would sound incomplete or unnatural in most contexts

You might omit them in a heading or label, such as:

• Formulario aduanero
• Declaración aduanera

But in a complete sentence, keeping el and la is the normal choice.

Because it is talking about two different things:

• the customs form
• the customs declaration

Even if it feels repetitive in English, this is completely normal in Spanish, especially in formal or official language.

If the context were already clear, Spanish could shorten it, for example:

• El formulario contiene la declaración

But the full sentence is not awkward at all.