Learn Hardcore German: Wir haben das Lied für den Chor oft proben müssen, aber jetzt klingt es gut. - We have had to rehearse the song for the choir many times, but now it sounds good.

The main action is müssen (a modal verb) + proben (to rehearse).
In German, modal verbs (müssen, können, wollen, dürfen, sollen, mögen) always use haben in the perfect tense, not sein.
sein is only used with:
- Verbs of movement or change of state (gehen, kommen, sterben, werden, etc.)
- The verbs sein and bleiben themselves.

So even though in English you say "we have had to rehearse", in German the structure is wir haben … müssen, always with haben.

Why is it proben müssen at the end and not something like müssen geprobt?

This is the “double infinitive” construction in German.

Pattern with a modal in the perfect:

haben/sein (conjugated) + … + main verb (infinitive)
- modal verb (infinitive)

So:

Wir haben das Lied … proben müssen.
- haben = auxiliary (perfect tense)
- proben = main verb (infinitive)
- müssen = modal verb (infinitive) at the very end

You do not say:

✗ Wir haben das Lied müssen geprobt.
✗ Wir haben das Lied gemusst proben.

The participle gemusst exists but is rarely used and normally only when there is no other verb:

Ich habe das nicht gewusst. (from wissen, not müssen)
Ich habe nicht gemusst. (rare and sounds odd in many contexts)

With a main verb like proben, the natural and standard structure is:

Wir haben [Object/Adverbials] proben müssen.

Why is there no zu before proben or müssen (no zu proben müssen)?

German modal verbs (müssen, können, wollen, etc.) take a bare infinitive without zu.

Wir müssen das Lied proben. (not: ✗ zu proben)
In the perfect: Wir haben das Lied proben müssen.

Similarly:

Ich will gehen. (not: ✗ zu gehen after a modal)
Wir können kommen.

You add zu with many other verbs:

Wir versuchen, das Lied zu proben.
Wir haben vergessen, das Lied zu proben.

But with modal + main verb, it’s always without zu.

Can the word oft be placed somewhere else in the sentence? Is this position the only correct one?

Oft (often) is fairly flexible. Your sentence:

Wir haben das Lied für den Chor oft proben müssen, aber jetzt klingt es gut.

Possible alternatives (all grammatically correct, with slightly different emphasis):

Wir haben das Lied oft für den Chor proben müssen.
– Emphasises das Lied being rehearsed often.
Wir haben oft das Lied für den Chor proben müssen.
– Emphasises that we have often had to do this whole action.
Wir haben das Lied für den Chor proben müssen – oft.
– Put oft at the end for extra emphasis (more spoken style).

All are acceptable; the original placement before proben müssen is perfectly natural and common.

Why is it für den Chor and not für dem Chor?

Because für is a preposition that always takes the accusative case in German.

für + den Chor (accusative masculine)
Not für dem Chor (which would be dative and is incorrect here)

Some common accusative-only prepositions:

für (for)
durch (through)
gegen (against)
ohne (without)
um (around / at [time])

So you must say:

für den Chor, für den Lehrer, für die Kinder, für das Kind, etc.

Why is it das Lied? What gender is Lied, and do I just have to memorize it?

Lied is neuter in German, so the definite article is das:

das Lied (singular)
die Lieder (plural)

Gender in German is largely lexical (built into the word), so in most cases, yes, you simply have to memorize it with its article:

das Lied
der Chor
die Stimme (voice)

There are some patterns (e.g. many nouns ending in -chen or -lein are neuter), but Lied just has to be learned as neuter: das Lied.

Why is it den Chor and not der Chor? Isn’t Chor masculine?

Yes, Chor is masculine, but the form of the article changes with the case.

Masculine definite article:

Nominative: der Chor (subject)
Accusative: den Chor (direct object)
Dative: dem Chor
Genitive: des Chores

Here, für forces the accusative, so you must use den:

für den Chor = for the choir (accusative masculine)

If Chor were the subject, you would say:

Der Chor klingt gut. – The choir sounds good.

Why does the sentence use klingt instead of a verb like hört or hört sich an?

The verb klingen means “to sound” (i.e. how something sounds).

Jetzt klingt es gut. – Now it sounds good.

Usage:

Die Musik klingt schön.
Deine Stimme klingt müde.
Das Lied klingt besser als vorher.

You could also use:

Jetzt hört es sich gut an. (more colloquial, also common)
Jetzt klingt das Lied gut. (more specific subject)

But:

hören by itself is mostly “to hear”:
- Ich höre das Lied. – I hear the song.

So klingen is exactly the right verb for “to sound (good / bad / strange, etc.)”.

Why is there a comma before aber?

In German, a comma is usually required before coordinating conjunctions like:

aber (but)
und (and) – in some cases
oder (or)
denn (for, because)

Here you have two independent clauses:

Wir haben das Lied für den Chor oft proben müssen
jetzt klingt es gut

They are connected by aber, so a comma is placed before aber:

…, aber jetzt klingt es gut.

This is similar to English, where a comma before but is very common:

We had to rehearse the song for the choir often, but now it sounds good.

Why is the ending … klingt es gut and not … klingt gut es?

In klingen + adjective structures, the pattern is:

Subject – Verb – Adjective
- Es klingt gut.
- Das Lied klingt schön.
- Der Chor klingt laut.

Es is the subject here, so it must come directly after the verb in a simple clause if nothing else is fronted:

Jetzt klingt es gut.

Gut is a predicate adjective (describing how “it” sounds) and goes after the verb:

Not: ✗ klingt gut es

Could I also say Wir mussten das Lied für den Chor oft proben instead of the perfect tense? What is the difference?

Yes, you can say:

Wir mussten das Lied für den Chor oft proben, aber jetzt klingt es gut.

Difference:

Perfekt (haben + infinitives: wir haben … proben müssen)
- More common in spoken German for past events.
- Feels slightly more conversational.
Präteritum (simple past: wir mussten … proben)
- Very common in written German (books, reports, etc.).
- In everyday speech, Präteritum is used mostly with a few verbs (war, hatte, wollte, konnte, musste, sollte, durfte).

Both versions are correct and mean the same thing in context:

Wir haben das Lied … proben müssen.
Wir mussten das Lied … proben.

The choice is mostly about style (spoken vs written) rather than meaning.