Wanneer ek honger is, maak ek gou 'n toebroodjie en sit die botter terug in die yskas.

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

So:

• Wanneer ek honger is = When I am hungry
• literally: When I hungry am

This is a very common pattern:

• as ek tyd het = if I have time
• omdat hy moeg is = because he is tired
• terwyl ons wag = while we wait

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

When the sentence begins with Wanneer ek honger is, that whole clause takes the first position. So the main clause must begin with the verb:

• Wanneer ek honger is, maak ek ...

If you started directly with the subject, you would get:

• Ek maak gou 'n toebroodjie ...

So after an opening clause or adverb, inversion happens:

• Vandag maak ek kos
• As ek honger is, maak ek kos

In Afrikaans, the normal way to say I am hungry is Ek is honger.

So honger behaves like an adjective here:

• Ek is honger = I am hungry
• Sy is moeg = She is tired
• Ons is dors = We are thirsty

This is different from some languages that use have for hunger or thirst. In Afrikaans, to be is the natural choice here.

Here gou means quickly or in a moment / just quickly.

In this sentence:

• maak ek gou 'n toebroodjie = I quickly make a sandwich

It often has a slightly informal, natural feel, like doing something without much delay.

Depending on context, gou can mean:

• quickly
• soon
• for a moment
• just quickly

But in this sentence, quickly is the best fit.

'n is the Afrikaans indefinite article, meaning a or an.

So:

• 'n toebroodjie = a sandwich

A few important things about 'n:

• it is always written with an apostrophe
• it is always lower-case, even at the beginning of a sentence
• in pronunciation, it is usually just a very weak vowel sound, like uh

For example:

• 'n Hond blaf. = A dog is barking.

Even at the start of the sentence, 'n stays lower-case.

Toebroodjie means sandwich.

It contains the diminutive ending -jie, which is very common in Afrikaans. Even though diminutives often literally mean something small, toebroodjie is simply the normal everyday word for sandwich. It does not necessarily mean a tiny sandwich.

So in this sentence:

• 'n toebroodjie = a sandwich

Yes. In Afrikaans, sit can mean put / place / set when it is used transitively.

So:

• sit die botter terug in die yskas = put the butter back in the fridge

This is different from English, where sit usually means to be seated. Afrikaans sit can do both jobs, depending on how it is used.

Compare:

• Ek sit op die stoel. = I am sitting on the chair.
• Ek sit die boek op die tafel. = I put the book on the table.

So here, sit is not sit down. It means place or put.

Terug means back.

So:

• sit die botter terug in die yskas = put the butter back in the fridge

It shows that the butter is being returned to where it was before.

You will often see terug used this way:

• gaan terug = go back
• kom terug = come back
• gee terug = give back

In this sentence, it adds the idea of returning the butter to the fridge after using it.

Because Afrikaans has one main definite article: die.

It does not change for:

• gender
• most noun types
• singular vs. plural

So:

• die botter = the butter
• die yskas = the fridge
• die kinders = the children

This is much simpler than languages that have different forms like the / le / la / les or der / die / das.

Afrikaans often uses in where English might use either in or into.

So:

• in die yskas can mean in the fridge
• with a verb like sit meaning put, it naturally gives the sense into the fridge

In other words, the verb already shows movement, so in is enough.

Here:

• sit die botter terug in die yskas = put the butter back in the fridge / into the fridge

Both are good English translations.

It can mean either, depending on context.

In isolation, this sentence often sounds like a general habit:

• Wanneer ek honger is, maak ek gou 'n toebroodjie ...
• Whenever I’m hungry, I quickly make a sandwich ...

But it can also refer to a specific situation:

• When I’m hungry, I quickly make a sandwich ...

Afrikaans present tense is often flexible like that, and context tells you whether it is a one-time situation or a repeated habit.

Yskas means fridge or refrigerator.

It is the normal everyday word in Afrikaans.

So:

• die yskas = the fridge

It is made up of:

• ys = ice
• kas = cupboard / cabinet

So historically it is something like ice-cabinet, but you should just learn it as the standard word for fridge.