Toe ek by die poskantoor aankom, het die posbode reeds vertrek.

In this sentence, toe is a conjunction introducing a time clause:

Toe ek by die poskantoor aankom = When I arrived at the post office

So here toe means when.

Afrikaans toe can also mean then, but that is a different use. Compare:

• Toe ek hom sien, groet ek hom. = When I see/saw him, I greet him
• Ek het hom gesien; toe het ek hom gegroet. = I saw him; then I greeted him

So the clue is that toe is followed by a whole clause.

Because Toe ek by die poskantoor aankom is a subordinate clause, and in Afrikaans subordinate clauses usually send the verb to the end.

So the order is:

• Toe = when
• ek = I
• by die poskantoor = at the post office
• aankom = arrive

This verb-final pattern is very common after conjunctions such as toe, omdat, dat, as, and terwyl.

After toe, Afrikaans very often uses the simpler verb form in a past-time clause, especially in normal narration:

• Toe ek daar aankom ... = When I arrived there ...

Even though it looks like a present-tense form, the meaning is past because toe already places the action in the past.

You may also hear or see Toe ek by die poskantoor aangekom het ..., which is grammatical, but it is often felt to be heavier or more explicit. The shorter Toe ek ... aankom is very natural.

Afrikaans main clauses follow a verb-second pattern. That means the finite verb must come in the second position.

Here, the entire toe-clause comes first:

Toe ek by die poskantoor aankom, ...

Because that whole clause takes the first position, the finite verb of the main clause must come next:

..., het die posbode reeds vertrek.

If the sentence started directly with the subject, you would get:

Die posbode het reeds vertrek.

So the fronted time clause causes the inversion.

Afrikaans often uses its normal perfect tense where English would use a past perfect.

So:

het die posbode reeds vertrek
is naturally understood as
the postman had already left

The time relationship is clear from the context:

1. I arrived
2. By that time, the postman was already gone

The word reeds helps make that sequence clear. Afrikaans often relies more on context than English does for this kind of past-before-past meaning.

Because vertrek begins with the inseparable prefix ver-, and verbs with prefixes like be-, ge-, her-, er-, ont-, and ver- normally do not take ge- in the perfect.

So:

• vertrek → het vertrek
• not het gevertrek

This is the same pattern you see in verbs like:

• betaal → het betaal
• ontmoet → het ontmoet
• verkoop → het verkoop

Reeds means already.

In this sentence:

het die posbode reeds vertrek = the postman had already left

Yes, you can often use al instead, and in everyday spoken Afrikaans al is often more common:

• ... het die posbode al vertrek.

The difference is mainly one of style:

• reeds = a bit more formal or written
• al = very common and conversational

Here by means at.

So:

• by die poskantoor = at the post office

This is different from:

• na die poskantoor = to the post office
• in die poskantoor = in/inside the post office

Since the sentence is talking about the place where the speaker arrived, by is the natural choice.

They are two forms of the same separable verb.

• In some positions, the verb stays together: aankom
• In a main clause, the prefix often separates: kom ... aan

Compare:

• Ek kom by die poskantoor aan. = I arrive at the post office.
• Toe ek by die poskantoor aankom ... = When I arrived at the post office ...
• om by die poskantoor aan te kom = to arrive at the post office

So English speakers often need to learn both shapes of the same verb.

Because Afrikaans normally writes compound nouns as a single word.

So:

• pos
  • kantoor → poskantoor
• pos
  • bode → posbode

This is very common in Afrikaans and much more systematic than in English. English might write similar ideas as two words, hyphenated, or one word, but Afrikaans usually prefers one solid compound.

Yes, that is a perfectly correct sentence, but it would mean simply:

The postman had already left / has already left

What you would lose is the time frame created by the first clause:

Toe ek by die poskantoor aankom

So the full sentence is doing two things:

1. setting the moment: when I arrived at the post office
2. telling what was already true at that moment: the postman had already left

That is why the full version sounds more complete.