Wir warten, bis die Kaffeebohnen kalt sind.

Because bis die Kaffeebohnen kalt sind is a subordinate clause.

In German, subordinate clauses are normally separated from the main clause with a comma:

• Wir warten, bis die Kaffeebohnen kalt sind.

The main clause is:

• Wir warten

The subordinate clause is:

• bis die Kaffeebohnen kalt sind

So the comma is required.

It comes at the end of the subordinate clause.

In German, when a clause is introduced by a subordinating conjunction like bis, the conjugated verb usually moves to the end:

• main clause: Wir warten
• subordinate clause: bis die Kaffeebohnen kalt sind

This is a very common pattern:

• Ich bleibe hier, bis du zurückkommst.
• Wir essen, nachdem er angekommen ist.

So sind is at the end because bis triggers subordinate-clause word order.

Here, bis is a subordinating conjunction meaning until.

It connects the main action to a time limit or endpoint:

• Wir warten → the main action
• bis die Kaffeebohnen kalt sind → the point in time when the waiting ends

So grammatically, bis introduces the clause that tells you how long the waiting continues.

Because die Kaffeebohnen is the subject of the subordinate clause, so it is in the nominative case.

Inside the clause bis die Kaffeebohnen kalt sind:

• die Kaffeebohnen = subject
• sind = verb
• kalt = predicate adjective

Also, Kaffeebohnen is plural, and the nominative plural definite article is die.

That is why you get:

• die Kaffeebohnen sind ...

not:

• den Kaffeebohnen (dative)
• der Kaffeebohnen (genitive)

Because Kaffeebohnen is plural.

• singular: die Kaffeebohne ist kalt
• plural: die Kaffeebohnen sind kalt

The verb has to agree with the subject in number, so plural Kaffeebohnen takes sind.

Because kalt is being used as a predicate adjective, not as an adjective directly before a noun.

Compare:

• die kalten Kaffeebohnen → adjective before noun, so it gets an ending
• die Kaffeebohnen sind kalt → adjective after sein, so no ending

After verbs like sein, werden, and bleiben, adjectives usually stay in their basic form:

• Der Kaffee ist heiß.
• Die Bohnen bleiben trocken.
• Das Wasser wird kalt.

So kalt has no ending here because it describes the subject after sind.

Because this sentence uses wait until, not wait for.

In English, these are different:

• We wait for the coffee beans.
• We wait until the coffee beans are cold.

German makes the same distinction:

• auf etwas/jemanden warten = to wait for something/someone
• warten, bis ... = to wait until ...

So:

• Wir warten auf den Bus. = We are waiting for the bus.
• Wir warten, bis die Kaffeebohnen kalt sind. = We wait until the coffee beans are cold.

That is why there is no auf in your sentence.

Yes. German very often makes compound nouns, and Kaffeebohnen is one of them.

It is made from:

• Kaffee = coffee
• Bohnen = beans

Together:

• Kaffeebohnen = coffee beans

This is completely normal in German. Also remember:

• Nouns are capitalized, so Kaffeebohnen starts with a capital letter.

German often uses the simple present where English uses either simple present or present progressive.

So:

• Wir warten can mean we wait or we are waiting, depending on context.

German does not usually need a separate -ing form the way English does.

So this sentence is completely natural in German, even if English might often say:

• We are waiting until the coffee beans are cold.

Yes, you could, and it is very similar, but the nuance is slightly different.

• bis die Kaffeebohnen kalt sind focuses on the state reached: until they are cold
• bis die Kaffeebohnen kalt werden focuses a bit more on the change/process: until they become cold

In many everyday situations, both are possible.
The version with sind is very natural because bis already implies the endpoint, so German often describes the resulting state.

Yes. You can move the bis clause to the front:

• Bis die Kaffeebohnen kalt sind, warten wir.

That still means the same thing.

A few things happen when you do this:

1. The subordinate clause still keeps the verb at the end:

   • bis die Kaffeebohnen kalt sind

2. The main clause then follows with the verb in second position:

   • warten wir

So both are correct:

• Wir warten, bis die Kaffeebohnen kalt sind.
• Bis die Kaffeebohnen kalt sind, warten wir.

The first version is probably the more neutral one here.

Not always.

Bis can be:

1. a conjunction, introducing a clause

   • Wir warten, bis die Kaffeebohnen kalt sind.

2. a preposition, followed by a time expression

   • Bis morgen!
   • Bis nächste Woche warten wir noch.

In your sentence, it is clearly a conjunction because it introduces a clause with its own subject and verb:

• die Kaffeebohnen ... sind