O carteiro deixa a carta na caixa do correio.

Because Portuguese nouns have grammatical gender.

• o is the masculine singular definite article
• a is the feminine singular definite article

So:

• o carteiro = the postman
• a carta = the letter

Portuguese also uses definite articles more often than English. In a normal sentence like this, leaving them out would sound unnatural.

Na is a contraction of em + a.

• em = in / on / at
• a = the for a feminine singular noun
• em + a = na

Since caixa is feminine singular, na caixa means in the box.

This kind of contraction is very common in Portuguese and is normally required.

For the same reason: Portuguese usually contracts de + o into do.

• de = of / from
• o = the
• de + o = do

So caixa do correio literally means box of the mail/post, which corresponds to mailbox / letter box.

Deixa is the 3rd person singular present tense of the verb deixar.

So it means:

• he/she leaves
• it leaves
• the postman leaves

Here, the subject is o carteiro, so deixa means the postman leaves.

Yes, deixar can mean both to leave and to let / allow. Context tells you which meaning is intended.

In this sentence, deixa a carta na caixa do correio, the structure is:

• person + verb + object + place

That strongly suggests leave / put: the postman leaves the letter in the mailbox.

If it meant let, you would usually see a different structure, often with another verb:

• Deixa-me falar. = Let me speak.

So here deixa clearly means leaves / puts / drops off.

It can mean either, depending on context.

In Portuguese, the simple present often expresses:

• a general fact
• a routine
• a habitual action
• sometimes a present action, especially in narration

So this sentence could mean:

• The postman leaves the letter in the mailbox as a general/habitual action, or
• in the right context, something happening now

If you want to make right now especially clear in European Portuguese, you often use estar a + infinitive:

• O carteiro está a deixar a carta na caixa do correio.

It means mailbox or letter box.

Literally, it is box of the mail/post:

• caixa = box
• correio = mail, post, postal service

Here correio is used as a mass noun, like English mail, so the singular is normal.

In everyday use, caixa do correio usually refers to the place where letters are delivered.

Because this is the normal, neutral Portuguese order:

Subject + Verb + Object + Place

• O carteiro = subject
• deixa = verb
• a carta = direct object
• na caixa do correio = location

This is the most straightforward way to say it. Portuguese can change word order for emphasis, but this version is the standard, unmarked one.

In both words, the x is pronounced like sh.

A rough English approximation is:

• deixa ≈ DAY-shuh
• caixa ≈ KAI-shuh

For European Portuguese specifically, the final a is usually weak, not a strong ah sound.

So:

• deixa sounds closer to DAY-shuh
• caixa sounds closer to KAI-shuh

Yes. Carteiro is the masculine form.

The feminine form is carteira:

• o carteiro = the male postman
• a carteira = the female postwoman / postal worker

One useful thing to remember: carteira can also mean wallet/purse in Portuguese, so context matters.

Yes, depending on the nuance.

• deixa = leaves / puts / drops off
• entrega = delivers
• põe = puts
• coloca = places / puts

So:

• O carteiro entrega a carta... focuses more on delivery
• O carteiro põe a carta... focuses more on putting
• O carteiro deixa a carta... is very natural for leaves the letter there

In this sentence, deixa is a good everyday choice.