Wir kühlen die Kaffeebohnen, bis der Temperaturfühler zwanzig Grad zeigt.

Because bis introduces a subordinate clause here: bis der Temperaturfühler zwanzig Grad zeigt.

In German subordinate clauses, the conjugated verb usually goes to the end. So:

• main clause: Wir kühlen die Kaffeebohnen
• subordinate clause: bis der Temperaturfühler zwanzig Grad zeigt

That is one of the most important word-order patterns in German.

Because the part starting with bis is a subordinate clause, and German normally separates subordinate clauses from the main clause with a comma.

So the comma in:

• Wir kühlen die Kaffeebohnen, bis der Temperaturfühler zwanzig Grad zeigt.

is required, not optional.

Die Kaffeebohnen is the direct object of kühlen.

The verb kühlen is transitive, so it takes an object in the accusative case:

• wir = subject
• kühlen = verb
• die Kaffeebohnen = direct object

Since Kaffeebohnen is plural, the article is die. In this case, plural nominative and plural accusative look the same, so the word order and verb tell you its role.

Because der Temperaturfühler is the subject of the subordinate clause.

In der Temperaturfühler zwanzig Grad zeigt, the sensor is the thing doing the showing, so it is in the nominative case:

• der Temperaturfühler = subject
• zeigt = verb
• zwanzig Grad = what it shows

If it were a direct object, you would expect den instead.

German often uses zeigen for instruments, displays, gauges, and thermometers.

So these are natural:

• Das Thermometer zeigt 20 Grad.
• Die Anzeige zeigt 20 Grad.
• Der Temperaturfühler zeigt 20 Grad.

In English, reads is often more natural, but German commonly uses zeigt in this context.

In temperature expressions, German normally uses Grad after numbers:

• 20 Grad
• 5 Grad
• minus 3 Grad

So zwanzig Grad is the normal form.

You may sometimes see Grad Celsius for extra clarity:

• zwanzig Grad Celsius

But just zwanzig Grad is very common when the temperature scale is understood from context.

When bis is used as a conjunction meaning until, yes: it introduces a subordinate clause, so the verb goes to the end.

Example:

• Wir warten, bis er kommt.
• Wir kühlen die Kaffeebohnen, bis der Temperaturfühler zwanzig Grad zeigt.

But bis can also appear in other structures, such as with zu:

• bis zu 20 Grad

That is not a subordinate clause, so the verb-final rule does not apply there.

Yes. German very often combines nouns into one compound word.

Here:

• Kaffee = coffee
• Bohne = bean
• Kaffeebohne = coffee bean
• Kaffeebohnen = coffee beans

This is completely normal in German. English often writes similar ideas as separate words, but German usually joins them.

Yes, you could, depending on nuance.

• kühlen = to cool
• abkühlen = to cool down

So:

• Wir kühlen die Kaffeebohnen sounds straightforward and neutral.
• Wir kühlen die Kaffeebohnen ab emphasizes bringing them down to a lower temperature.

In technical or process-oriented language, both can work, but kühlen is perfectly natural here.

Because German very often uses the present tense for:

• routines
• instructions
• scheduled actions
• process descriptions

So Wir kühlen die Kaffeebohnen... can mean something like:

• We cool the coffee beans...
• We are cooling the coffee beans...
• We will cool the coffee beans...

The exact English translation depends on context, but German does not need a special future form here.

It is natural in a technical context. A Temperaturfühler is a temperature sensor or probe.

In everyday speech, Germans might more often say:

• Thermometer if they mean a thermometer
• Anzeige if they mean the display
• Messgerät if they mean the measuring device

So Temperaturfühler sounds a bit technical, but it is not wrong.

Yes. German can use bis with different structures.

With a full clause:

• ... bis der Temperaturfühler zwanzig Grad zeigt.

With a noun phrase, often using bis zu or a different structure depending on meaning:

• bis morgen
• bis zum Abend

But for this sentence, the speaker wants to express the exact condition that must be reached, so a full clause is very natural.

It breaks down like this:

• Wir = subject
• kühlen = verb
• die Kaffeebohnen = object
• bis der Temperaturfühler zwanzig Grad zeigt = subordinate clause

So the pattern is:

main clause + subordinate clause

And inside the subordinate clause, the structure is:

• der Temperaturfühler = subject
• zwanzig Grad = object/complement of what is shown
• zeigt = verb at the end

That is a very common German sentence pattern.