Ek trek my jas uit wanneer ek by die huis aankom.

Because the full verb is uittrek, which is a separable verb in Afrikaans.

In a main clause, the conjugated part goes earlier in the sentence and the separable part goes to the end:

• Ek trek my jas uit. = I take off my coat.

This is similar to separable verbs in Dutch and a bit like English phrasal verbs such as take off.

You can think of it like this:

• dictionary form: uittrek
• main clause: trek ... uit

So trek and uit belong together, even though other words come between them.

Because wanneer ek by die huis aankom is a subordinate clause introduced by wanneer.

In Afrikaans, subordinate clauses usually send the verb to the end. So:

• ek kom by die huis aan = I arrive at home
• wanneer ek by die huis aankom = when I arrive at home

Notice that the separable verb aankom stays together at the end in the subordinate clause, instead of splitting into kom ... aan.

So the pattern is:

• main clause: Ek kom by die huis aan
• subordinate clause: wanneer ek by die huis aankom

Here wanneer means when.

Yes, in many everyday situations Afrikaans speakers also use as where English would use when, especially for habitual situations:

• Ek trek my jas uit wanneer ek by die huis aankom.
• Ek trek my jas uit as ek by die huis aankom.

Both are understandable and natural.
Very roughly:

• wanneer = clearly when
• as = often if or when, depending on context

For learners, wanneer is often the safer choice when you specifically mean when.

In Afrikaans, possessive adjectives are simple and do not change for gender or case.

So:

• my jas = my coat
• jou jas = your coat
• sy jas = his coat
• haar jas = her coat

Unlike some languages, Afrikaans does not change the form depending on whether the noun is masculine, feminine, singular, or plural.

So my jas is just the normal way to say my coat.

Not by itself. Trek has a broader meaning, often related to pull, draw, or put on / wear / move depending on context.

Examples:

• trek = pull / draw
• klere aantrek = put clothes on
• klere uittrek = take clothes off

So in this sentence, it is really the combination uittrek that gives the meaning take off.

That is why Ek trek my jas uit means I take off my coat, not literally just I pull my coat.

By die huis means at home or at the house.

In this sentence, the person has already arrived, so the meaning is location, not direction.

• by die huis = at home / at the house
• huis toe = homeward / to home

Compare:

• Ek gaan huis toe. = I am going home.
• Ek is by die huis. = I am at home.
• Ek trek my jas uit wanneer ek by die huis aankom. = I take off my coat when I arrive at home.

So huis toe would not fit as well here, because aankom already expresses arriving.

That is just a normal difference between the languages.

Afrikaans often says:

• by die huis = at home
• in die huis = in the house

Even when English uses home without an article, Afrikaans often uses die huis.

So you should not translate word for word too rigidly.
In this sentence, by die huis aankom is a natural Afrikaans way to express arrive home / arrive at home.

Not really in the same way. Huis toe kom means something more like come home or come toward home, focusing on movement toward home.

Your original sentence uses by die huis aankom, which focuses on arriving at home.

A more natural alternative would be:

• Ek trek my jas uit as ek by die huis kom.
• Ek trek my jas uit wanneer ek tuiskom.

Both are common.
But huis toe kom is less suitable if you want the meaning when I arrive home in this structure.

Because each clause has its own subject.

The sentence has two clauses:

1. Ek trek my jas uit
2. wanneer ek by die huis aankom

Both clauses have ek as the subject, so Afrikaans states it in both places, just as English does:

• I take off my coat when I arrive home.

You cannot usually omit the second ek here.

The sentence has:

• a main clause first
• then a subordinate clause introduced by wanneer

Structure:

• Ek = subject
• trek = finite verb
• my jas = object
• uit = separable particle
• wanneer ek by die huis aankom = time clause

So the pattern is roughly:

Subject + verb + object + separable particle + subordinate clause

Inside the subordinate clause, the verb goes to the end:

• wanneer + subject + place + verb

That is why it is:

• wanneer ek by die huis aankom

not:

• wanneer ek aankom by die huis

Yes. In a main clause, the separable verb splits:

• Ek kom by die huis aan. = I arrive at home.

In a subordinate clause, it usually stays together:

• wanneer ek by die huis aankom

So both forms belong to the same verb:

• infinitive / dictionary form: aankom
• main clause: kom ... aan
• subordinate clause: aankom

This is exactly the same kind of pattern as uittrek becoming trek ... uit in the main clause.

Yes. A very natural shorter version would be:

• Ek trek my jas uit as ek tuiskom.

Here tuiskom means come home / get home.

So these are close in meaning:

• Ek trek my jas uit wanneer ek by die huis aankom.
• Ek trek my jas uit as ek tuiskom.

The original sentence is perfectly correct, but tuiskom is often more compact and very common in everyday Afrikaans.