Die posbode sê dat die brief môre by daardie adres sal aankom.

Because die is the normal definite article in Afrikaans for the.

Unlike languages such as German or Dutch, Afrikaans does not change the article for gender. So you use die for all definite singular nouns and also for plural nouns in most cases.

• die posbode = the postman / the mail carrier
• die brief = the letter

So seeing die more than once in the same sentence is completely normal.

Dat means that.

It introduces a subordinate clause after sê:

• Die posbode sê = The postman says
• dat die brief môre by daardie adres sal aankom = that the letter will arrive at that address tomorrow

So dat works very much like English that in a sentence such as He says that the letter will arrive tomorrow.

This is because Afrikaans word order changes in a subordinate clause introduced by dat.

In the clause:

• dat die brief môre by daardie adres sal aankom

the finite verb sal goes toward the end, and the main verb aankom comes last.

So the pattern is roughly:

• dat
  • subject + other information + sal
    • main verb

This is different from normal English word order, where you would say will arrive together earlier in the clause.

Because in Afrikaans subordinate clauses, the main verb often goes to the end.

Here, aankom is the infinitive/main lexical verb, and sal is the auxiliary showing future meaning. In a dat-clause, Afrikaans typically puts the verb cluster at the end:

• dat ... sal aankom

This is one of the most important word-order patterns for English speakers to get used to.

Sal means will.

It marks the future:

• Die brief sal aankom = The letter will arrive

In your sentence, because it is inside a dat-clause, it appears near the end:

• dat die brief ... sal aankom

So sal is the future auxiliary, just like English will.

Aankom means arrive.

Yes, it comes from a separable verb pattern:

• base idea: kom aan in some contexts
• dictionary form: aankom

In main clauses, separable verbs can split:

• Die brief kom môre aan. = The letter arrives / is arriving tomorrow.

But with another verb like sal, the infinitive usually stays together:

• Die brief sal môre aankom.
• ... dat die brief môre sal aankom.

So learners often notice both patterns:

• split in some main clauses
• unsplit in infinitive form after an auxiliary

Here by means something like at.

With aankom, Afrikaans often uses by to show the place where something arrives:

• by die huis = at the house
• by die adres = at the address

English often says arrive at a place, and that matches Afrikaans by quite well here.

Na usually means to / toward, focusing more on direction or movement toward somewhere, while by fits the endpoint where the item arrives.

Daardie means that in the sense of that one over there or that particular.

So:

• daardie adres = that address

It contrasts with:

• hierdie adres = this address

For a learner, it is useful to remember:

• hierdie = this
• daardie = that

Here it means tomorrow.

Afrikaans môre can be confusing because it is also seen in greetings such as Goeie môre = Good morning.

But in this sentence, the time meaning is clearly tomorrow:

• die brief môre ... sal aankom = the letter will arrive tomorrow

So context tells you which meaning is intended.

The accent marks help show pronunciation and sometimes distinguish words more clearly in writing.

• sê = say
• môre = tomorrow / morning

For an English-speaking learner, the key point is that the accents are part of the normal written form of many Afrikaans words and should be learned with the word.

You do not need to overthink them grammatically here; they mainly help with spelling and pronunciation.

If you remove the reporting structure and make the future statement stand on its own, you get:

• Die brief sal môre by daardie adres aankom.

That is a useful comparison because it shows the word-order change.

Main clause:

• Die brief sal môre by daardie adres aankom.

Subordinate clause after dat:

• ... dat die brief môre by daardie adres sal aankom.

So the meaning stays the same, but the position of sal changes.

Afrikaans has largely lost grammatical gender in the way English learners usually think about it.

That is why you do not need to learn different forms like the masculine noun, the feminine noun, and so on for the article. In this sentence, both nouns simply take die:

• die posbode
• die brief

So compared with Dutch or German, Afrikaans is simpler here.

The first part is very similar:

• Die posbode sê = The postman says

The main difference comes after dat. English usually keeps a more familiar order:

• that the letter will arrive tomorrow

Afrikaans changes the order inside the subordinate clause:

• dat die brief môre by daardie adres sal aankom

So for an English speaker, the biggest structural point is:

• after dat, the verb pattern moves toward the end of the clause.