Wanneer die ketel warm is, weet my oupa dat die tee gereed is.

Because wanneer introduces a subordinate clause. In Afrikaans, subordinate clauses usually send the finite verb to the end.

So:

• Die ketel is warm = the kettle is warm
• Wanneer die ketel warm is = when the kettle is warm

This same pattern happens with many other words such as dat, omdat, as, and terwyl.

Afrikaans main clauses normally follow a verb-second pattern. That means the finite verb comes second.

Here, the whole opening clause Wanneer die ketel warm is counts as the first element. So the verb of the main clause must come next:

• Wanneer die ketel warm is, weet my oupa ...

If you started directly with the subject, you would get the more basic order:

• My oupa weet ...

So yes, this is a kind of inversion after a fronted clause.

Dat means that and introduces another subordinate clause:

• weet my oupa dat die tee gereed is
• my grandfather knows that the tea is ready

In English, that is often optional, but in Afrikaans dat is very commonly kept.

For the same reason as in wanneer die ketel warm is: dat also introduces a subordinate clause, and subordinate clauses usually place the finite verb at the end.

Compare:

• Die tee is gereed = the tea is ready
• dat die tee gereed is = that the tea is ready

Because die is the normal definite article in Afrikaans, meaning the.

A useful thing for English speakers is that Afrikaans does not change the article for grammatical gender the way Dutch or German does. In everyday use:

• die ketel = the kettle
• die tee = the tea

The same article is used across nouns, which makes things simpler.

My is the possessive determiner meaning my, and it goes directly before the noun:

• my oupa = my grandfather
• my huis = my house

It does not change for gender or case. Also, Afrikaans does not use an apostrophe here.

A learner sometimes confuses my with forms like myne, but myne is a stand-alone form, not the one used before a noun.

Oupa is the normal everyday word for grandpa / grandfather. It sounds warm and natural.

There is also grootvader, but that is more formal, more literary, or less common in ordinary speech. In daily Afrikaans, oupa is very common.

Gereed means ready in the sense of prepared or in a state of readiness.

You will also see klaar, which can mean ready, but very often also means finished / done.

So:

• Die tee is gereed = the tea is ready
• Die tee is klaar can also work in some contexts, but it may feel slightly more like the tea is done/finished

In this sentence, gereed is a very good choice.

It can be understood as either, depending on context.

• In a specific situation, it means when
• In a general repeated situation, it can feel like whenever

So this sentence could mean either:

• at the moment when the kettle is warm, my grandfather knows the tea is ready
• whenever the kettle is warm, my grandfather knows the tea is ready

Context decides which reading is stronger.

Yes, in standard Afrikaans writing, a fronted subordinate clause is normally followed by a comma before the main clause.

So the punctuation here is standard:

• Wanneer die ketel warm is, weet my oupa dat die tee gereed is.

The comma helps show the boundary between the subordinate clause and the main clause.