As die posbus vol is, hou ek die brief tot môre in my sak.

In Afrikaans, as can mean if and sometimes when, depending on context.

In this sentence, it is most naturally understood as if: a condition is being given—if the mailbox is full, I keep the letter in my pocket until tomorrow.

If the speaker means this as a repeated, normal situation, English might also translate it as when. So the exact nuance comes from context.

Because as introduces a subordinate clause, and in Afrikaans subordinate clauses usually send the finite verb to the end.

So:

• die posbus is vol = main clause word order
• as die posbus vol is = subordinate clause word order

This is very common in Afrikaans:

• Ek weet dat hy moeg is.
• Omdat dit koud is, bly ons binne.
• As die posbus vol is, ...

Because the sentence begins with the subordinate clause As die posbus vol is. After that, the main clause follows, and Afrikaans keeps the verb in the second position overall.

So the structure is:

• As die posbus vol is, hou ek die brief ...

If the main clause stood alone, it would be:

• Ek hou die brief tot môre in my sak.

But once something else comes first, the verb comes before the subject:

• Môre hou ek die brief in my sak.
• As die posbus vol is, hou ek die brief in my sak.

This is one of the most important word-order patterns in Afrikaans.

Here hou means keep or hold onto.

Afrikaans hou is a very common verb with several meanings, including:

• keep / hold
• like in expressions such as Ek hou van koffie = I like coffee

In this sentence, hou ek die brief ... in my sak means I keep the letter in my pocket/bag.

Yes. Vol means full, and here it is a predicate adjective, just like English full in the mailbox is full.

So:

• die posbus is vol = the mailbox is full

Afrikaans adjectives often behave simply after is and do not change form here.

Compare:

• ’n vol posbus = a full mailbox
• Die posbus is vol = The mailbox is full

Because the sentence starts with a subordinate clause:

• As die posbus vol is, ...

In standard written Afrikaans, a comma is used to separate that opening clause from the main clause.

So the comma helps show the structure:

• subordinate clause first
• main clause second

Die is the Afrikaans definite article, meaning the.

So:

• die posbus = the mailbox
• die brief = the letter

Afrikaans uses die for singular and plural definite nouns, unlike English, which changes only the noun:

• die brief = the letter
• die briewe = the letters

In this sentence, both nouns are specific, so die is used.

Tot môre means until tomorrow.

• tot = until / till
• môre = tomorrow (and in some contexts morning)

So in this sentence, hou ek die brief tot môre means the speaker will keep the letter until the next day.

The circumflex in môre is part of the normal spelling and helps mark pronunciation.

Because in Afrikaans, a possessive like my normally replaces the article.

So you say:

• my sak = my pocket / my bag
• not die my sak

This works just like English:

• my bag
• not the my bag

It can mean either, depending on context.

Common meanings of sak include:

• pocket
• bag
• sack

In this sentence, in my sak will often be understood as in my pocket, because someone is temporarily keeping a letter on their person. But if the situation suggests a handbag, school bag, or similar container, bag could also fit.

Yes, that is possible, and the meaning is basically the same.

However, As die posbus vol is, hou ek die brief tot môre in my sak is often more natural if you want to emphasize the condition first.

So the choice is about focus:

• As die posbus vol is, ... = emphasizes the condition
• Ek hou die brief ... as die posbus vol is = starts with the main action

Both are grammatical.

Yes. Die posbus is vol is a complete main clause meaning the mailbox is full.

In the full sentence, that idea becomes part of a subordinate clause after as, so the word order changes:

• main clause: Die posbus is vol.
• subordinate clause: As die posbus vol is, ...

That contrast is very useful to notice when learning Afrikaans word order.