In hierdie geval wag ons.

Because In hierdie geval has been moved to the front of the sentence.

In Afrikaans main clauses, when something other than the subject comes first, the finite verb usually comes next. This is often called inversion.

• Neutral order: Ons wag.
• With a fronted phrase: In hierdie geval wag ons.

So the pattern is:

• [Time/place/manner/etc.] + verb + subject

This is very similar to Dutch and a bit less common in everyday English grammar.

It is a prepositional phrase meaning in this case.

It gives the context for the statement. In other words, it tells you that the action applies in this particular situation.

Breakdown:

• in = in
• hierdie = this
• geval = case

So in hierdie geval functions like an adverbial phrase at the start of the sentence.

Hierdie means this or these, depending on context.

Here it means this, because geval is singular.

• hierdie geval = this case
• hierdie gevalle = these cases

It is not die because die usually means the.
So:

• die geval = the case
• hierdie geval = this case

Also, hierdie does not change for grammatical gender, because Afrikaans does not use grammatical gender the way some other languages do.

Yes, you can.

• Ons wag in hierdie geval = We wait in this case
• In hierdie geval wag ons = In this case, we wait

Both are grammatical. The difference is mainly focus and style:

• Ons wag in hierdie geval starts with the subject and sounds more straightforward.
• In hierdie geval wag ons emphasizes the circumstance first.

So fronting in hierdie geval makes the sentence feel a little more marked or contrastive, like you are distinguishing this situation from another one.

Because hierdie already acts as the determiner.

In English, this already fills the same slot as the:

• this case
  not this the case

The same is true in Afrikaans:

• hierdie geval
  not hierdie die geval

So nothing is missing here.

Wag means wait.

In this sentence it is used without an object, so it simply means to wait.

Examples:

• Ons wag. = We wait / We are waiting.
• Hulle wag buite. = They are waiting outside.

If you want to say wait for someone/something, Afrikaans usually uses vir:

• Ons wag vir hom. = We are waiting for him.
• Ek wag vir die bus. = I am waiting for the bus.

So in your sentence, wag stands on its own because no person or thing being waited for is mentioned.

Yes, ons is the normal Afrikaans word for we.

It is also used for us, so the same form can cover both subject and object functions.

Examples:

• Ons wag. = We wait.
• Hy sien ons. = He sees us.

In your sentence, ons is the subject, even though it comes after the verb because of inversion.

Not in the same way English does.

Wag ons in this sentence can often correspond to English we wait or we are waiting, depending on context.

Afrikaans often uses the simple present where English might use either:

• the simple present
• the present progressive

So Ons wag can mean:

• We wait
• We are waiting

The exact meaning usually comes from context.

A rough English-friendly guide is:

in HEER-dee khuh-FAL vakh ons

A few useful notes:

• hierdie is roughly HEER-dee
• g in geval and wag is the throaty Afrikaans g/ch sound, not the English g in go
• v in Afrikaans is often pronounced close to English f
• wag is therefore often heard more like vakh

This is only an approximation, but it helps with the general shape of the sentence.

Yes, very common.

Afrikaans frequently puts an adverbial phrase first and then places the verb before the subject. This is a normal feature of the language.

Examples:

• Vandag werk ek. = Today I work / Today I’m working.
• By die huis eet ons. = At home we eat.
• In hierdie geval wag ons. = In this case we wait.

So this sentence shows a very typical Afrikaans pattern, not an unusual exception.