Die hond hardloop terug huis toe wanneer hy die hek oop sien.

Because wanneer introduces a subordinate clause in Afrikaans.

In a main clause, Afrikaans normally keeps the finite verb in second position:

• Hy sien die hek oop.

But after a subordinating conjunction such as wanneer, the verb usually moves to the end:

• wanneer hy die hek oop sien

So the final sien is a normal word-order pattern, not something special about this verb alone.

Huis toe is a very common Afrikaans expression meaning homeward / to home.

It works a lot like English go home, where English also drops the article and does not say go to the home in ordinary speech.

So:

• huis toe = home
• skool toe = to school
• werk toe = to work
• bed toe = to bed

That is why you do not see na here.

Here toe is marking direction: movement toward a place.

So in huis toe, the idea is not just home as a location, but toward home.

This use of toe is very common after places or destinations:

• kerk toe
• stad toe
• dorp toe

It is best learned as a pattern: place + toe for movement toward that place.

Because they do two different jobs:

• terug = back / back again
• huis toe = home / toward home

So together they express back home.

If you removed terug, the sentence would still say the dog runs home, but it would lose the idea of returning. If you removed huis toe, it would just say the dog runs back, without stating the destination.

Afrikaans does not use it the way English does.

For animals, Afrikaans often uses:

• hy for he
• sy for she

Even when English would naturally say it, Afrikaans often uses a personal pronoun instead. With a dog, hy is very normal, especially if the animal is being treated as an individual or if its sex is known.

So hy here is not strange at all.

These two patterns are similar, but not identical.

• die oop hek sien = see the open gate
  This describes the gate as an open gate.

• die hek oop sien = see the gate open
  This focuses on the gate being in an open state.

In this sentence, oop is functioning like a complement describing the object die hek after the act of seeing. English does something similar in see the gate open.

So die hek oop sien is a very natural Afrikaans structure.

Often, yes.

In everyday Afrikaans, as can sometimes be used where English would say when, especially in less formal speech. Wanneer is a clearer, more specifically temporal word meaning when.

So:

• wanneer hy die hek oop sien = definitely when he sees the gate open
• as hy die hek oop sien can also work in many contexts

A learner is usually safest using wanneer when the meaning is plainly temporal.

Because Afrikaans does not automatically place a comma before every subordinate clause the way English sometimes does stylistically.

A sentence like this is commonly written without a comma:

• Die hond hardloop terug huis toe wanneer hy die hek oop sien.

If the subordinate clause comes first, a comma is more likely:

• Wanneer hy die hek oop sien, hardloop die hond terug huis toe.

So the lack of a comma here is normal.

Yes, hardloop is normally written as one word and means run.

By contrast:

• loop = walk / go / move on foot
• hardloop = run

So if the idea is that the dog is running, hardloop is the right choice.

Yes, absolutely:

• Wanneer hy die hek oop sien, hardloop die hond terug huis toe.

This is a very useful word-order point in Afrikaans.

When a different element comes first in a main clause, the finite verb still has to stay in second position. So after the fronted wanneer clause, the next part is:

• hardloop die hond not
• die hond hardloop

That inversion is completely normal in Afrikaans.

Because terug naturally comes before the destination phrase in this kind of movement expression.

So:

• hardloop terug huis toe = idiomatic and natural

Here the sentence first gives the idea of returning, then gives the destination.

While word order can sometimes vary for emphasis, terug huis toe is the standard, natural order a learner should copy.

Yes. The verbs hardloop and sien are in the present tense.

In Afrikaans, the present tense can describe:

• something happening now
• a habitual action
• a general pattern
• a vivid narrative

So this sentence can easily mean something like a regular or typical action: whenever the dog sees the gate open, it runs back home. Context tells you whether it is a one-time event or a repeated behavior.