O código postal está correto.

Portuguese uses two verbs for to be: ser and estar.

Here, está is used because it describes a state or condition: the postal code is correct at this moment / in this context.

• O código postal está correto. = The postal code is correct.
• estar is very common with adjectives like correto, errado, aberto, fechado, when you are checking a condition or result.

If you said é correto, it would sound more like it is correct in general / it is the correct thing / it is valid as a principle, which is not the usual idea here.

O means the.

So:

• o = the
• código postal = postal code

Portuguese usually uses definite articles more often than English does, so o is completely natural here.

In Portuguese, the normal order is:

• noun + adjective

So:

• código = code
• postal = postal

Together: código postal = postal code

This is very common in Portuguese. English often puts one noun or modifier before another noun, but Portuguese usually puts the descriptive word after it.

Again, Portuguese usually places adjectives after the noun.

So:

• código postal correto = correct postal code

In your sentence, the adjective comes after the verb:

• O código postal está correto.

That is the most natural word order.

Because correto agrees with código, which is a masculine singular noun.

• o código → masculine singular
• so the adjective must be correto

If the noun were feminine, you would use correta:

• A morada está correta. = The address is correct.

If it were plural:

• Os códigos postais estão corretos.

The accents mainly show which syllable is stressed.

• código → stress on có
• está → stress on tá

They also help distinguish words in spelling.

A rough pronunciation guide for European Portuguese:

• código ≈ KOH-dee-goo
• postal ≈ push-TAHL / pohsh-TAHL (very roughly)
• está ≈ sh-TAH
• correto ≈ koo-REH-too (roughly; European Portuguese often reduces unstressed vowels)

These are only approximations for an English speaker.

A rough European Portuguese pronunciation is:

O código postal está correto.
≈ u KOH-di-gu push-TAHL shtah ku-REH-tu

A few important points:

• O often sounds like u
• unstressed vowels are often reduced in European Portuguese
• es- before a consonant often sounds like sh-, so está sounds close to shtá
• the final -o often sounds like u

This is one reason European Portuguese can sound much less phonetic to English speakers than it looks at first.

Yes, certo is possible in many situations, and it can also mean correct / right.

However, correto often sounds a bit more precise or formal when you are validating information.

So:

• O código postal está correto. = very natural when checking data
• O código postal está certo. = also understandable and natural in many contexts

If you are entering details on a form or checking an address, correto is an especially good choice.

Normally, no. In a full sentence, you would usually say:

• O código postal está correto.

Leaving out the article sounds incomplete in standard Portuguese, unless you are using headings, labels, notes, or very telegraphic language.

For example:

• on a form label: Código postal
• in a full statement: O código postal está correto.

You usually keep the same word order and change the intonation:

• O código postal está correto? = Is the postal code correct?

In writing, you just add a question mark. Portuguese does not normally require inversion like English does.

You could also ask more explicitly:

• O código postal está correto ou errado? = Is the postal code correct or wrong?

Yes. In Portugal, código postal is the normal expression.

That is the standard term you will see in everyday life, on forms, websites, and addresses.

An English speaker should remember that:

• postal code = código postal
• not zip code, which is specifically American English

So this sentence sounds completely natural in European Portuguese.

The plural would be:

Os códigos postais estão corretos.
= The postal codes are correct.

Notice all the changes:

• o → os
• código → códigos
• postal → postais
• está → estão
• correto → corretos

Portuguese shows agreement in several places, so plural sentences often change more words than in English.