El código postal completa la dirección postal.

Because código is a masculine singular noun in Spanish, so it takes the masculine singular definite article el.

• el código postal = the postal code / zip code
• la dirección postal = the mailing address

The article has to match the noun in gender and number:

• el código
• los códigos

Spanish nouns have grammatical gender. Here:

• código is masculine → el código
• dirección is feminine → la dirección

There is no special meaning difference here; it is just part of how each noun works in Spanish, and the article must agree with it.

Código postal is the standard Spanish term for postal code in Latin America. In many contexts, an English speaker may think of it as zip code, although ZIP code is specifically the U.S. term.

So:

• código postal = postal code / zip code

In Latin America, this is the normal formal expression.

In Spanish, most adjectives come after the noun. So:

• código postal = postal code
• dirección postal = postal address / mailing address

This is one of the basic differences from English, where adjectives usually come before nouns.

Because it modifies two different nouns:

• código postal = postal code
• dirección postal = postal address / mailing address

Even though English might not always repeat postal in a natural translation, Spanish often keeps the adjective with each noun for clarity.

Here, completa is the verb completar in the third-person singular present:

• completar = to complete
• completa = completes

So El código postal completa la dirección postal literally means The postal code completes the mailing address.

In context, it suggests that the postal code is the final piece needed to make the address complete.

Because the subject is el código postal, which is singular.

Spanish verbs must agree with the subject:

• El código postal completa... = The postal code completes...
• Los códigos postales completan... = The postal codes complete...

So the singular subject requires the singular verb form completa.

You can tell from the structure of the sentence:

• El código postal = subject
• completa = verb
• la dirección postal = direct object

If completa were an adjective, the sentence would need a different structure, such as:

• La dirección está completa = The address is complete

So in your sentence, completa clearly means completes, not complete.

Because postal is an adjective that has the same form for masculine and feminine singular.

So you get:

• el código postal
• la dirección postal

And in the plural:

• los códigos postales
• las direcciones postales

Many Spanish adjectives work this way: they do not change for gender, only for number.

Because adjectives in Spanish must agree in number with the noun they describe.

Singular:

• código postal
• dirección postal

Plural:

• códigos postales
• direcciones postales

Since postal ends in a consonant, you usually add -es to make it plural:

• postal → postales

Yes, often you can. Dirección by itself usually means address, and in many situations the mailing idea is understood from context.

But dirección postal is more specific:

• dirección = address
• dirección postal = mailing address / postal address

So adding postal makes it clear that you mean the address used for mail.

The given order is the most neutral and natural:

• El código postal completa la dirección postal.

Spanish word order is somewhat flexible, but changing it can sound more marked or emphasize something different. For example:

• La dirección postal la completa el código postal.

That version is grammatical, but it is less neutral and sounds more emphatic or literary. For a learner, the original sentence is the best pattern to use.

Because the stress falls on the first syllable: CÓ-di-go.

Without the accent, Spanish spelling rules would make you stress it differently. The written accent tells you the correct pronunciation.

So:

• código → stress on có

Accent marks in Spanish are important because they often show where the word is stressed.

No, not normally. In Spanish, common nouns and adjectives are not capitalized unless they begin a sentence.

So the normal form is:

• El código postal completa la dirección postal.

You would not write:

• El Código Postal completa la Dirección Postal.

unless you had some very unusual stylistic reason.

Yes. Completar is commonly used to mean to complete, to fill out, or to make something complete.

Depending on context, you might see:

• completar un formulario = to fill out a form
• completar la información = to complete the information
• completar la dirección = to complete the address

So this sentence uses the verb in a normal, understandable way.