Learn Hardcore German: Meine Schwester spielt Geige im Schulchor, und der Musiker schreibt extra einfache Stücke für sie. - My sister plays violin in the school choir, and the musician writes especially simple pieces for her.

Sie spielt die Geige.
- more specific: she is playing the violin (a particular one, or at this moment)
- often used for a concrete situation.
Sie spielt Geige.
- more general: she plays violin (as an ability / activity, like a hobby or skill)

In your sentence, the idea is about her general role in the choir, so spielt Geige is natural.

What exactly is im Schulchor, and why im instead of in dem?

im is a standard contraction of in dem:

in (preposition) + dem (dative masculine/neuter article) → im

Schulchor is a compound noun:

die Schule (school) + der Chor (choir) → der Schulchor (school choir)

Because Chor is masculine (der Chor), Schulchor is also masculine (der Schulchor).
After in with a location (where something happens), German uses the dative:

in dem Schulchor (in the school choir) → contracted to im Schulchor.

So im Schulchor literally means in the school choir.

Why is there a comma before und: ..., und der Musiker schreibt ...?

In German, when und connects two full main clauses, a comma is required:

Meine Schwester spielt Geige im Schulchor, und der Musiker schreibt extra einfache Stücke für sie.
- Clause 1: Meine Schwester spielt Geige im Schulchor.
- Clause 2: Der Musiker schreibt extra einfache Stücke für sie.

Each part has its own subject and finite verb, so they are two independent clauses.

In modern English, the comma before and is often optional; in German, in this situation the comma is obligatory.

Why is it der Musiker and not ein Musiker?

Der Musiker is the definite article (the musician), suggesting a specific, known person:

probably the choir’s regular accompanist, arranger, or music teacher.

Using ein Musiker would introduce a new, indefinite person:

... und ein Musiker schreibt extra einfache Stücke für sie.
→ “and some musician writes especially easy pieces for her” (someone, not clearly specified).

In typical context, we assume everyone knows which musician is meant in the school/choir setting, so der Musiker is more natural.

What does extra mean here? Is it “additional” or “especially”?

In this sentence, extra means especially / particularly, not “additional”:

extra einfache Stücke ≈ especially easy pieces / extra-easy pieces

So the idea is: the musician is deliberately making the pieces easier than usual for her. It does not mean “additional pieces” (like more pieces on top); that would require a different wording, e.g. zusätzliche Stücke.

Why is it einfache Stücke and not something like einfachen Stücke?

Adjective endings in German depend on the article (or lack of one), plus gender/number/case.

Stücke here is:
- plural (from das Stück → die Stücke)
- accusative (direct object of schreibt)
- with no article, only adjectives (extra
  - einfach)

For plural, accusative, with no article, the strong adjective ending is -e:

extra einfache Stücke

Compare:

Er schreibt die einfachen Stücke.
- definite article die, so einfachen
Er schreibt einfache Stücke.
- no article, so einfache

So einfache is correct here.

What does Stücke mean here? Is it “pieces” in the musical sense?

Yes. das Stück is a very general word meaning piece, bit, item, play, etc. In a musical context, (Musik)stücke or just Stücke typically means pieces of music or musical works:

ein Stück für Geige und Klavier – a piece for violin and piano
leichte Stücke – easy (music) pieces

So einfache Stücke in this sentence are easy musical pieces.

Why is sie in für sie “her” and not “she” or “they”?

The pronoun sie is ambiguous in German; it can mean:

she (nominative singular)
her (accusative singular)
they (nominative/accusative plural)
formal you (Sie with capital S)

Here, the preposition für always takes the accusative, so sie must be accusative:

für sie → for her (referring back to meine Schwester)

It cannot mean she here, because für requires an object, not a subject.
Capitalization helps with the formal Sie, but here it’s lowercase sie, so it’s her.

Why is der Musiker schreibt ... and not something like a future tense (“will write”)?

German often uses the present tense (Präsens) where English might use:

present continuous (“is writing”)
or a future form (“will write”).

Der Musiker schreibt extra einfache Stücke für sie. can be:

literal present: The musician writes especially easy pieces for her.
or contextually: The musician is writing / will write especially easy pieces for her.

If you really want to emphasize the future, you can say:

Der Musiker wird extra einfache Stücke für sie schreiben.

But in everyday language, the simple present is very common for planned or ongoing future actions, especially when context makes the time clear.

Why is Schulchor one word instead of Schul Chor or Schul-Chor?

German favors compound nouns written as a single word:

die Schule
- der Chor → der Schulchor

Writing it as Schul Chor would be wrong; Schul-Chor with a hyphen is sometimes used for stylistic reasons (headlines, posters), but the standard spelling is Schulchor.

German often joins nouns together like this:

das Haus
- die Tür → die Haustür (front door)
die Schule
- das Fach → das Schulfach (school subject)

Why are Schwester, Geige, Schulchor, Musiker, and Stücke all capitalized?

In German, all nouns are capitalized, no matter where they appear in the sentence:

Meine Schwester spielt Geige im Schulchor, und der Musiker schreibt extra einfache Stücke für sie.

Here:

Schwester – noun (sister)
Geige – noun (violin)
Schulchor – noun (school choir)
Musiker – noun (musician)
Stücke – noun (pieces)

This is a key difference from English, where only proper nouns and sentence starts are capitalized.

Is the word order Der Musiker schreibt extra einfache Stücke für sie fixed, or can it be changed?

The given order is very natural:

Subject: Der Musiker
Verb: schreibt
Direct object: extra einfache Stücke
Prepositional phrase: für sie

You can move some elements for emphasis, but the finite verb (schreibt) must stay in second position in a main clause. Examples:

Der Musiker schreibt für sie extra einfache Stücke.
(emphasis slightly more on “for her”)
Für sie schreibt der Musiker extra einfache Stücke.
(strong emphasis: for her he is doing this)

What you cannot do is move the verb away from second position:

✗ Der Musiker extra einfache Stücke für sie schreibt. (wrong as a main clause)

So the original word order is both correct and neutral.