Learn Hardcore German: Im Schulorchester spielt meine Schwester Flöte. - In the school orchestra my sister plays the flute.

Orchester is neuter (das Orchester), and after in with a location (no movement), German uses the dative case, so in dem Orchester → im Orchester.

So im Schulorchester literally means in the school orchestra.
You cannot write in Schulorchester here; it would be ungrammatical. The full form in dem Schulorchester is correct but sounds a bit formal or heavy in everyday language, so im Schulorchester is preferred.

Why does the sentence start with Im Schulorchester instead of Meine Schwester?

German word order is more flexible than English. A typical neutral order would be:

Meine Schwester spielt Flöte im Schulorchester.

Here, we move the place phrase im Schulorchester to the front to emphasize where she plays:

Im Schulorchester spielt meine Schwester Flöte.

In German main clauses, the finite verb must be in second position (the “verb-second rule”). The first position can be many things: subject, time, place, object, etc. Here, Im Schulorchester is in first position, so the verb spielt must come next, and the subject meine Schwester comes after that.

Why is the verb spielt before the subject meine Schwester? That sounds like a question in English.

In English, “Plays my sister the flute” would sound like a question, but German works differently.

In a main clause, German requires:

Some element in first position (here: Im Schulorchester).
The conjugated verb in second position (here: spielt).
The rest of the information (subject, objects, etc.) afterwards (here: meine Schwester Flöte).

So:

Im Schulorchester (position 1)
spielt (position 2, the finite verb)
meine Schwester Flöte (everything else)

It is not a question because:

The verb is second, not first.
There is no question word and no rising intonation is implied in writing.

A yes/no question in German would put the verb first:

Spielt meine Schwester im Schulorchester Flöte?

What cases are used in this sentence: Im Schulorchester spielt meine Schwester Flöte.?

The cases are:

Im Schulorchester → dative
- Preposition in with a static location uses dative: in dem Schulorchester → im Schulorchester.
- Schulorchester itself stays the same in all cases; the article changes (here, dem → im).
meine Schwester → nominative (subject)
- The person doing the action (my sister) is the subject, so nominative.
- Feminine nominative singular of meine.
Flöte → accusative (direct object), but without an article
- The thing being played is the direct object.
- Feminine singular nouns often look the same in nominative and accusative without an article, so Flöte doesn’t show the case by itself here.

Why is it meine Schwester and not meiner Schwester?

Because meine Schwester is the subject of the sentence and must be in the nominative case.

Feminine forms of mein:

Nominative singular: meine Schwester (subject)
Accusative singular: meine Schwester (direct object) – same form
Dative singular: meiner Schwester
Genitive singular: meiner Schwester

We need the nominative because she is doing the action:

Meine Schwester spielt Flöte. → nominative subject

Meiner Schwester would be dative, e.g.:

Im Schulorchester gefällt es meiner Schwester.
(In the school orchestra, my sister likes it. / It pleases my sister in the school orchestra.)

Why doesn’t Flöte have an article? Why not eine Flöte or die Flöte?

With musical instruments, German often uses a bare noun (no article) after spielen when you mean the general skill:

Er spielt Klavier. – He plays piano.
Sie spielt Gitarre. – She plays guitar.
Meine Schwester spielt Flöte. – My sister plays the flute.

So Flöte without an article is completely normal and idiomatic.

You can say die Flöte spielen, but that often sounds more like a concrete, specific instrument:

Sie spielt die Flöte, die sie zum Geburtstag bekommen hat.
(She plays the flute that she got for her birthday.)

In your sentence, Flöte without an article simply means she plays the instrument in general, in that orchestra.

Can I also say Meine Schwester spielt Flöte im Schulorchester? Is there a difference?

Yes, that is perfectly correct:

Meine Schwester spielt Flöte im Schulorchester.

Both sentences are grammatical:

Im Schulorchester spielt meine Schwester Flöte.
Meine Schwester spielt Flöte im Schulorchester.

Differences:

The version starting with Meine Schwester is more neutral and “English-like” in order (Subject–Verb–Object–Place).
The version starting with Im Schulorchester gives a bit more emphasis to the location (the school orchestra) by placing it first.

In normal conversation, both word orders are common; the choice is mostly about what you want to highlight.

Why are Schulorchester, Schwester, and Flöte capitalized?

In German, all nouns are capitalized. That’s a core rule of the spelling system.

In this sentence:

Schulorchester – noun (neuter)
Schwester – noun (feminine)
Flöte – noun (feminine)

So they must all start with a capital letter. This helps readers quickly identify nouns in a sentence.

Is Schulorchester one word? How is it formed?

Yes, Schulorchester is written as one word. German very often forms compound nouns by joining two (or more) nouns:

Schule (school) + Orchester (orchestra) → Schulorchester (school orchestra)

In compounds:

Only the last part determines the gender and plural:
- das Orchester → das Schulorchester, plural: die Schulorchester
The first part(s) specify or narrow down the meaning.

So Schulorchester is a specific type of Orchester that belongs to or is connected with a Schule.

How would the sentence look in different tenses, like past or with a modal verb?

Using the same structure, you can change tense or add a modal:

Simple past (Präteritum) of spielen (regular verb):

Im Schulorchester spielte meine Schwester Flöte.
(In the school orchestra, my sister played the flute.)

Present perfect (Perfekt):

Im Schulorchester hat meine Schwester Flöte gespielt.
(In the school orchestra, my sister has played / played the flute.)

With a modal verb, e.g. “can” (können):

Im Schulorchester kann meine Schwester Flöte spielen.
(In the school orchestra, my sister can play the flute.)

Note that with a modal verb or in the perfect tense, the conjugated verb still stays in second position, and the other verb goes to the end.