Wanneer die werk klaar is, wil ons net op die bank ontspan.

Because wanneer introduces a subordinate clause, and in Afrikaans the finite verb usually moves to the end of a subordinate clause.

So:

• main clause: Die werk is klaar
• subordinate clause: Wanneer die werk klaar is

This is very similar to other Afrikaans subordinators such as as, omdat, and dat.

Afrikaans is a verb-second language in main clauses. That means the finite verb usually comes in the second position.

Here, the first position is taken by the whole time clause:

Wanneer die werk klaar is,

So the verb must come next:

wil ons net op die bank ontspan

If you start directly with the subject, then you would get:

Ons wil net op die bank ontspan wanneer die werk klaar is.

Both are fine; the word order changes because of what comes first.

In this sentence, wanneer means when in the sense of at the time that.

It is used to introduce a time clause:

• Wanneer die werk klaar is = when the work is finished

In many everyday sentences, Afrikaans speakers may also use as where English uses when, depending on context and style. But wanneer is perfectly natural and clear here.

Klaar means finished, done, or ready, depending on context.

In die werk klaar is, klaar is not the main verb. The finite verb is is.
So structurally, it works like:

• Die werk is klaar = the work is finished

Here:

• die werk = subject
• is = verb
• klaar = predicate/adjective-like complement

Afrikaans often uses klaar this way.

Die is the common definite article in Afrikaans, used for the.

So:

• die werk = the work
• die bank = the couch/sofa/bench, depending on context

Unlike English, Afrikaans does not change the article for gender.
It is simply die for definite singular and plural nouns in most cases.

Because ontspan depends on the modal verb wil.

After modal verbs such as:

• kan
• moet
• mag
• sal
• wil

Afrikaans normally uses the bare infinitive, without te.

So:

• Ons wil ontspan = we want to relax

not:

• Ons wil te ontspan

Net here means just, simply, or only in a soft, everyday sense.

So wil ons net op die bank ontspan gives the idea:

• we just want to relax on the couch
• we simply want to relax on the couch

It adds a feeling of that’s all we want to do.

Be aware that net can have slightly different shades depending on context, including only.

Op means on.

So:

• op die bank = on the couch / on the sofa

This is the normal preposition for sitting or lying on a couch.
English speakers should note that bank in Afrikaans often means sofa/couch/bench, not just a financial bank.

Because the sentence contains the modal verb wil.

In Afrikaans main clauses with a modal verb, the finite verb comes early and the main infinitive usually goes to the end:

• Ons wil ontspan
• Sy kan kom
• Hulle moet werk

So in your sentence:

• wil = finite/modal verb
• ontspan = main infinitive at the end

Everything else fits in between:

wil ons net op die bank ontspan

Yes. That is also correct.

Compare:

• Wanneer die werk klaar is, wil ons net op die bank ontspan.
• Ons wil net op die bank ontspan wanneer die werk klaar is.

The meaning is basically the same, but the emphasis changes a little.

• Starting with Wanneer die werk klaar is highlights the time condition first.
• Starting with Ons wil sounds a bit more neutral or direct.

Here wil mainly means want to.

So wil ons ... ontspan means we want to relax.

Sometimes learners confuse wil with English future expressions, but in Afrikaans the usual future helper is sal, not wil.

Compare:

• Ons wil ontspan = we want to relax
• Ons sal ontspan = we will relax

So in this sentence, it is about desire or intention, not simple future.

Here werk means work in the uncountable sense: the task, the job, the work that needs to be done.

So:

• die werk = the work

It is singular in form here.
If you meant separate jobs or works, the wording would be different, but in this sentence die werk is the natural choice.

A helpful rough breakdown is:

• Wanneer = when
• die werk = the work
• klaar is = finished is
• wil = want
• ons = we
• net = just
• op die bank = on the couch
• ontspan = relax

So very literally:

When the work finished is, want we just on the couch relax.

That literal version sounds wrong in English, of course, but it helps show the Afrikaans structure:

1. subordinate clause with verb at the end
2. main clause with the finite verb in second position
3. infinitive at the end because of the modal wil