Learn Hardcore German: In der Nebenbeschäftigung lerne ich viel über Geduld, was mir auch im Hauptberuf hilft. - In the side job I learn a lot about patience, which also helps me in my main job.

The preposition in can take either accusative (direction: into) or dative (location: in/within). Here it means in/within the side job, so it uses dative:

in + der Nebenbeschäftigung → in the side job (location, feminine dative)

You can’t say im Nebenbeschäftigung because:

im = in dem (dative masculine/neuter),
but Beschäftigung is feminine, so its dative article is der, not dem.

So the correct forms are:

in der Nebenbeschäftigung (feminine dative)
im Hauptberuf = in dem Hauptberuf (masculine dative; see below)

Why is the word order In der Nebenbeschäftigung lerne ich and not In der Nebenbeschäftigung ich lerne?

German main clauses follow the verb‑second rule: the finite verb must be in second position in the sentence, no matter what comes first.

Here, the first element is the prepositional phrase In der Nebenbeschäftigung, so the verb has to come next:

In der Nebenbeschäftigung (1st position)
lerne (2nd position – the finite verb)
ich (rest of the clause)

You could also say:

Ich lerne in der Nebenbeschäftigung viel über Geduld …

Both are correct. Fronting In der Nebenbeschäftigung puts emphasis on where this learning happens.

What exactly does über Geduld mean here, and why is über used?

In this context, über means “about / concerning”:

viel über Geduld lernen = to learn a lot about patience

When über means about (a topic), it takes the accusative case. Here there’s no article (Geduld is used as an uncountable noun), so you don’t see the case marking, but structurally it is:

über (die) Geduld → accusative (about patience)

Why is it viel über Geduld instead of viele über Geduld?

Here viel is an adverb modifying the verb lerne:

Ich lerne viel über Geduld.
= I learn a lot about patience.

You only use viele when it directly modifies a countable plural noun:

viele Bücher – many books
viele Fehler – many mistakes

In the sentence, we’re not saying “many X about patience”; we’re saying “I learn a lot (in general) about patience”. So the correct form is viel (adverb), not viele (adjective before plural nouns).

In was mir auch im Hauptberuf hilft, what does was refer to, and why not die?

Here was is a relative pronoun that refers not to a single noun, but to the whole idea of the previous clause:

In der Nebenbeschäftigung lerne ich viel über Geduld,
→ the whole fact that I learn a lot about patience there
…, was mir auch im Hauptberuf hilft.
→ which also helps me in my main job.

So was refers to “that (fact)” / “that which”, i.e. the entire preceding statement.

If you wanted to refer specifically to the noun Geduld (patience), you’d use die (feminine relative pronoun):

Ich lerne viel über Geduld, die mir auch im Hauptberuf hilft.
= I learn a lot about patience, which also helps me in my main job.
(Here, patience itself is what helps.)

In the original sentence, was is chosen because it points to the whole situation of learning about patience, not just the noun Geduld by itself.

Why is there a comma before was?

German uses a comma to separate all subordinate clauses, including relative clauses.

…, was mir auch im Hauptberuf hilft.

This was‑clause explains or comments on the entire first clause. It’s grammatically a subordinate clause, so a comma before was is mandatory in standard German.

Why is it mir instead of mich in was mir auch im Hauptberuf hilft?

The verb helfen (to help) in German always takes the dative case, not the accusative:

jemandem helfen – to help someone (dative)

So you say:

Es hilft mir. – It helps me. (dative)
Er hilft dir. – He helps you.

Never mich/dich after helfen.

In the sentence:

… was mir auch im Hauptberuf hilft.
= … which also helps me in my main job.

mir is the dative form of ich, required by helfen.

In was mir auch im Hauptberuf hilft, why is hilft at the end of the clause?

This is because was mir auch im Hauptberuf hilft is a subordinate clause (introduced by the relative pronoun was).

In German subordinate clauses, the finite verb goes to the end:

… was mir auch im Hauptberuf hilft.
(relative pronoun – other elements – verb)

So the pattern is:

was (subordinating element)
mir auch im Hauptberuf (middle field)
hilft (finite verb at the end)

Why do we say im Hauptberuf but in der Nebenbeschäftigung?

This is mostly about grammatical gender and the resulting dative forms.

Hauptberuf = masculine noun
- Dative singular: dem Hauptberuf
- in dem Hauptberuf → contracted to im Hauptberuf
Nebenbeschäftigung = feminine noun
- Dative singular: der Nebenbeschäftigung
- in der Nebenbeschäftigung (no contraction possible)

So:

im Hauptberuf = in dem Hauptberuf (correct for masculine)
in der Nebenbeschäftigung (correct for feminine; im is impossible here)

Both phrases use dative because they indicate location (in/within), not direction.

What’s the difference between Nebenbeschäftigung, Nebenjob, Hauptberuf, and Beruf?

Beruf
- General word for profession / occupation.
- Mein Beruf ist Lehrer. – My profession is teacher.
Hauptberuf
- Your primary/main job – the one that defines your professional status or main income.
- Im Hauptberuf bin ich Ingenieur. – As my main job, I’m an engineer.
Nebenbeschäftigung
- Literally side activity / secondary employment.
- Slightly more formal/neutral; can include freelance work, part‑time work, or any secondary professional activity alongside the main job.
Nebenjob
- Colloquial, very common; means side job / part‑time job on the side.
- Often used for students or employees who have a small extra job in addition to something else.

In many contexts, Nebenbeschäftigung and Nebenjob overlap, but Nebenbeschäftigung sounds more formal/administrative, while Nebenjob is more everyday speech. The sentence you gave uses the more neutral/formal Nebenbeschäftigung.