Learn Hardcore German: Den Leuten, denen ich früher kaum zuhören konnte, höre ich jetzt konzentriert zu. - The people whom I could hardly listen to before I now listen to attentively.

Leute is plural.
The article den shows dative plural.
The verb zuhören always takes a dative object (you “listen to someone” → dative).

If you put that dative object (den Leuten) in first position for emphasis, it must keep its dative form:

Neutral order: Ich höre den Leuten zu.
Emphatic order: Den Leuten höre ich zu.

So den is required by case, not by word order position.

What case is “den Leuten”, and why does “zuhören” use that case?

Den Leuten is in the dative plural.

The verb zuhören means “to listen (attentively) to someone” and in German it takes its object in the dative, not in the accusative:

Ich höre dem Lehrer zu. – I listen to the teacher.
Wir hören den Leuten zu. – We listen to the people.

So any person you “listen to” with zuhören will be in the dative.

Why is it “denen” and not “die” in “denen ich früher kaum zuhören konnte”?

Denen is a relative pronoun referring back to den Leuten (“the people to whom …”).

Leute is plural.
Because of zuhören, the pronoun inside the relative clause must again be dative plural.

Relative pronouns in dative plural are:

denen (for all genders in plural; there is no die in dative plural).

So you get:

Den Leuten, denen ich früher kaum zuhören konnte, …
= The people to whom I could hardly listen in the past …

If you used die, that would be nominative or accusative plural, which does not match the verb zuhören inside the relative clause.

How is the case of the relative pronoun “denen” determined?

For a relative pronoun in German you have to look inside the relative clause, not at its role in the main clause.

Identify what role the pronoun plays in its own clause:
… denen ich früher kaum zuhören konnte
Here, zuhören needs a dative object → “I could hardly listen to them”.
The gender/number comes from the antecedent (Leuten, plural).
The case comes from the verb in the relative clause (zuhören → dative).

So: plural + dative → denen.

Why is the order “Den Leuten … höre ich … zu” and not simply “Ich höre den Leuten … zu”?

German main clauses have the finite verb (conjugated verb) in second position (the “V2 rule”). Everything else can be moved around for emphasis.

Neutral order would be:

Ich (position 1) höre (position 2) den Leuten, denen …, jetzt konzentriert zu.

If you want to emphasize the people, you move Den Leuten, denen ich früher kaum zuhören konnte into position 1:

Den Leuten, denen ich früher kaum zuhören konnte (position 1)
höre (position 2)
ich jetzt konzentriert zu.

The verb höre must stay in second position, so the subject ich moves behind it. This is normal and very common in German.

Why is it “höre … zu” and not just one word like “zuhöre” here?

Zuhören is a separable verb:

Infinitive: zuhören
Prefix: zu
Verb stem: hören

In a main clause:

The conjugated part (höre, hörst, hört, …) goes to position 2.
The separable prefix (zu) goes to the very end of the clause.

So:

Ich höre zu. – I listen.
Den Leuten … höre ich jetzt konzentriert zu.

You only write it together as zuzuhören / zuzuhören in infinitive forms or at the end of certain subordinate clauses, e.g.:

…, weil ich ihnen konzentriert zuhören will.
…, ohne ihnen zuzuhören.

Where do “jetzt” and “konzentriert” belong, and can they move?

Both are adverbs:

jetzt = now (time)
konzentriert = in a concentrated way, attentively (manner)

The neutral order of adverbs is often time–manner–place. Here we only have time and manner:

Ich höre den Leuten jetzt konzentriert zu.

You can move them somewhat, but they must stay inside the “midfield” (between the verb in second position and the stuff at the very end, including zu):

Ich höre jetzt den Leuten konzentriert zu.
Ich höre den Leuten konzentriert jetzt zu. (possible, but less usual)
Den Leuten … höre ich jetzt konzentriert zu. (as in your sentence)

You cannot split the prefix wrongly:

❌ … höre ich jetzt zu konzentriert. (this sounds like “too concentrated” and is misleading)
❌ … höre ich zu jetzt konzentriert. (ungrammatical)

The zu of zuhören must be the last element.

What’s the difference between “zuhören” and just “hören”?

hören = to hear (perceive sound); can also mean “to listen to” in some contexts but is more neutral.
zuhören = to listen attentively to someone; implies active attention.

Case usage:

hören usually takes an accusative object:
- Ich höre die Musik. – I hear the music.
- Ich höre den Hund bellen. – I hear the dog bark.
zuhören takes a dative object and is almost always about people (or sources of speech):
- Ich höre dir zu. – I listen to you.
- Ich höre den Leuten zu. – I listen to the people.

Your sentence specifically contrasts “hardly listening” in the past vs. “listening attentively now”, so zuhören fits perfectly.

What exactly does “kaum” mean in “ich früher kaum zuhören konnte”?

Kaum means “hardly / barely / scarcely”.

In ich früher kaum zuhören konnte, it modifies the ability to listen:

ich konnte kaum zuhören
= I could hardly listen (I almost didn’t listen at all; very little attention).

Nuances compared to other options:

kaum: suggests very little of something actually happened.
fast nicht: “almost not” – slightly closer to “basically not at all.”

So: kaum zuhören → you did listen a bit, but very poorly or without attention.

Why is “konnte” at the end of the relative clause, and where does “zuhören” go there?

Inside the relative clause „denen ich früher kaum zuhören konnte“ we have:

A modal verb: konnte (preterite of können).
A full verb: zuhören.

In German subordinate clauses (including relative clauses), the finite verb goes to the end of the clause. With a modal + infinitive, the order is:

Infinitive (full verb)
Modal (finite)

So:

…, denen ich früher kaum zuhören konnte.
literally: “… to whom I earlier hardly to-listen could.”

You cannot say:

❌ …, denen ich früher kaum konnte zuhören. (wrong order)

What is the function of the commas in this sentence?

The commas isolate the relative clause:

Den Leuten, [denen ich früher kaum zuhören konnte], höre ich jetzt konzentriert zu.

Rules:

A relative clause in German is always set off by commas.
It directly follows the noun it describes (den Leuten).
The second comma closes the relative clause and returns you to the main clause.

Without commas it would be hard to see where the additional information about den Leuten begins and where it ends.

Could this idea be expressed in a simpler or more neutral word order in German?

Yes. A more straightforward version (same meaning, less emphasis on the people) would be:

Ich höre den Leuten, denen ich früher kaum zuhören konnte, jetzt konzentriert zu.

Changes:

The subject ich is now in first position (neutral).
Den Leuten … is immediately after the verb, like in a basic sentence.
The relative clause and adverbs remain the same.

Even simpler, leaving out the relative clause:

Ich höre den Leuten jetzt konzentriert zu.
– I listen to the people attentively now.

The original version just highlights those people by moving Den Leuten, denen ich früher kaum zuhören konnte to the front.