Learn Hardcore German: Vor dem Auftritt proben alle im Chor noch einmal, damit die Stimmen sicher klingen. - Before the performance everyone in the choir rehearses once more so that the voices sound confident.

Here it’s a time point: before the performance → no movement, just a point in time. So vor takes the dative case, and masculine dative singular of der is dem:

Nominative: der Auftritt
Dative: dem Auftritt

So: Vor dem Auftritt … = Before the performance …

Why does vor take the dative case here? I learned some prepositions can take both dative and accusative.

vor is one of the so‑called two-way prepositions (Wechselpräpositionen): an, auf, hinter, in, neben, über, unter, vor, zwischen.

With location / position / time point → dative
- vor dem Haus (in front of the house)
- vor dem Auftritt (before the performance)
With movement towards a place → accusative
- Sie stellen sich vor das Haus. (They move to stand in front of the house.)

Talking about time (before the performance) is treated like a “static” situation, so dative is used: vor dem Auftritt.

Why is the verb proben in second position: Vor dem Auftritt proben alle im Chor … and not Vor dem Auftritt alle im Chor proben …?

German main clauses follow the verb-second (V2) rule:

The conjugated verb must be the second element in the sentence.
Only one element (word or phrase) may stand in front of the verb.

Here:

Vor dem Auftritt = one prepositional phrase (first element)
proben = conjugated verb (must be second)
alle im Chor
- noch einmal = rest of the clause

So the basic structure is:

Vor dem Auftritt (1st element)
proben (2nd element, the finite verb)
alle im Chor noch einmal (everything else)

You could say Alle im Chor proben vor dem Auftritt noch einmal, but then Alle im Chor is the first element and proben still stays in second position. The V2 rule is what you must keep; the fronted element is flexible.

What exactly is the subject of the sentence? Is it alle or der Chor?

The subject here is alle (everyone / all of them).

proben is conjugated in 3rd person plural (sie proben).
alle is a plural pronoun-like word, so it matches that plural verb.
im Chor is just a prepositional phrase giving more information (in the choir), not the subject.

So in the main clause Vor dem Auftritt proben alle im Chor …:

Subject: alle
Verb: proben
Adverbial phrase: im Chor

What is the difference between proben and üben? Could I say üben here instead?

Both proben and üben can be translated as to practice, but they’re used differently:

proben = to rehearse, usually as a group, especially for a performance (theatre, choir, band, orchestra).
- Wir proben für das Konzert. – We’re rehearsing for the concert.
üben = to practice a skill or a piece, often individually.
- Ich übe Klavier. – I’m practising piano.
- Er übt seine Rolle. – He’s practising his role.

In the sentence, it’s a choir preparing for a performance, so proben is the natural and idiomatic choice.
Üben here would sound off; it wouldn’t be what a native speaker says in this context.

What does noch einmal mean exactly? How is it different from wieder?

Both can be translated as again, but there’s a nuance:

noch einmal = once more / one more time, usually a single additional repetition.
- Mach das noch einmal. – Do that one more time.
wieder = again in a more general sense, often meaning back to how it was before or repeatedly / once again.
- Er ist wieder krank. – He’s sick again.
- Wir sehen uns wieder. – We’ll see each other again.

In … proben alle im Chor noch einmal, the idea is:
They rehearse one more time before the performance.
So noch einmal fits better than wieder, which could sound more like they are once again rehearsing (in general).

What does the conjunction damit do in this sentence, and how does it affect word order?

damit introduces a purpose clause: it means roughly so that / in order that.

Main clause: Vor dem Auftritt proben alle im Chor noch einmal, …
Purpose clause: damit die Stimmen sicher klingen.
→ They rehearse in order that the voices sound secure/confident.

Grammatically:

damit introduces a subordinate clause.
In subordinate clauses, the conjugated verb goes to the end.

So the normal main-clause order die Stimmen klingen sicher becomes:

damit die Stimmen sicher klingen

Here, klingen (the finite verb) is at the end because of damit.

Why is the verb klingen at the end of the clause damit die Stimmen sicher klingen?

In German, subordinate clauses introduced by words like dass, weil, wenn, obwohl, damit place the conjugated verb at the end.

Pattern:

Main clause: Die Stimmen klingen sicher. (verb in 2nd position)
Subordinate clause: …, damit die Stimmen sicher klingen. (verb at the end)

So:

damit → marks a subordinate (dependent) clause
Therefore, klingen must move to the end: … sicher klingen.

This verb-final word order is a key feature of German subordinate clauses.

What does sicher mean in damit die Stimmen sicher klingen? Does it mean "safe"?

Literally, sicher can mean safe or secure, but in the context of music and performing, it more often means:

confident
steady / stable
reliable, not shaky or uncertain

So die Stimmen klingen sicher suggests that the singers:

sing accurately (correct notes, rhythms, entrances), and
sound confident and not hesitant.

It’s not about safety in the physical sense; it’s about confidence and reliability in the sound.

Why is it die Stimmen? What does Stimmen mean here?

Stimme is a feminine noun: die Stimme = voice.

Singular: die Stimme – the voice
Plural: die Stimmen – the voices

In this sentence, die Stimmen refers to the singing voices of the people in the choir.

So die Stimmen = the voices (of the choir members), not votes (although Stimme can also mean vote in other contexts).

Why is it im Chor and not in den Chor? What is im exactly?

im is a contraction of in dem:

in (preposition) + dem (dative article) → im

Chor is masculine: der Chor (the choir). With in:

Dative (location: in the choir, a state/place) → in dem Chor → im Chor
Accusative (movement into the choir) → in den Chor

Here, the meaning is everyone who is *in the choir, i.e. people who belong to it. That’s a static “location,” so *dative is used:

im Chor = in dem Chor = in the choir (as members of it)

Why is there a comma before damit?

In German, you must place a comma before a subordinate clause, and damit introduces such a clause.

Structure:

Main clause: Vor dem Auftritt proben alle im Chor noch einmal,
Subordinate clause: damit die Stimmen sicher klingen.

Rule:
Whenever you introduce a clause with dass, weil, wenn, obwohl, damit, während, bevor, etc., you separate it with a comma from the main clause.

So the comma before damit is obligatory, not optional.