Learn Hardcore German: Die Leserin findet vor allem die ruhige Stärke der Hauptfigur beeindruckend. - The female reader finds above all the calm strength of the main character impressive.

So Die Leserin specifically tells us the reader is female.
If you wanted a gender-neutral term, you’d need a different solution (e.g. die lesende Person, or modern written forms like Leser*innen, Leserinnen und Leser, etc.).

Why is it Die Leserin and not Eine Leserin?

Die Leserin uses the definite article (die) and refers to a specific female reader that the speaker and listener presumably know from context (e.g. “the reader of this novel,” “the reader we’re talking about”).

If you say Eine Leserin, you mean a (certain) reader, but she is not yet specifically defined.

Die Leserin findet … beeindruckend.
→ The (particular) female reader finds … impressive.
Eine Leserin findet … beeindruckend.
→ A female reader (some reader) finds … impressive.

Which part of the sentence is the subject and which part is the object?

The sentence is:

Die Leserin findet vor allem die ruhige Stärke der Hauptfigur beeindruckend.

Subject: Die Leserin
→ She is doing the action (she “finds” something).
Finite verb (conjugated verb): findet (3rd person singular of finden)
Object (accusative): die ruhige Stärke der Hauptfigur
→ This is what she finds impressive.
Predicate adjective / complement: beeindruckend
→ This says how or in what way she finds that object.
Adverbial: vor allem (“above all, especially”)
→ Gives extra information about the scope or emphasis.

So the core structure is: Die Leserin (subject) findet (verb) die ruhige Stärke der Hauptfigur (object) beeindruckend (predicate adjective).

How can I tell that die ruhige Stärke is accusative?

Look at this part:

die ruhige Stärke der Hauptfigur

The verb finden often takes a direct object in the accusative:

etwas finden = to find something

Since Die Leserin is the subject (nominative), the next main noun phrase after the verb findet is likely the accusative object.

For feminine nouns, nominative and accusative both use die:

Nominative singular feminine: die Stärke
Accusative singular feminine: die Stärke

So the form die Stärke looks the same in both cases.
We know it’s accusative here because:

Leserin is already the subject (nominative).
finden needs a direct object → that will be accusative: die ruhige Stärke.

What case is der Hauptfigur, and why does it look like that?

der Hauptfigur here is genitive singular feminine, meaning “of the main character”.

The pattern for a typical feminine noun like Hauptfigur is:

Nominative: die Hauptfigur
Accusative: die Hauptfigur
Dative: der Hauptfigur
Genitive: der Hauptfigur

So der Hauptfigur can be either dative or genitive, depending on context.

Here it must be genitive, because it follows another noun:

die ruhige Stärke der Hauptfigur
→ the quiet strength of the main character

This is a classic genitive attribute: one noun (Stärke) “owns” or is defined by another (Hauptfigur).

Could I say von der Hauptfigur instead of der Hauptfigur? What’s the difference?

Yes, you could say:

die ruhige Stärke von der Hauptfigur

This is grammatically possible, but compared to die ruhige Stärke der Hauptfigur, it sounds:

more colloquial and spoken-style
often a bit heavier / wordier

Genitive (der Hauptfigur) is:

more concise
more typical for written German, especially in narration or formal style

So:

die ruhige Stärke der Hauptfigur
→ neutral, natural in written German, slightly more elegant.
die ruhige Stärke von der Hauptfigur
→ tends to sound more casual or less stylistically polished, though still understandable.

Why is it ruhige Stärke and not ruhigen Stärke or ruhiger Stärke?

ruhige is the correctly declined adjective here.

We have:

article: die (definite article)
case: accusative
gender/number: feminine singular
noun: Stärke

With a definite article plus adjective plus feminine singular noun in accusative, the adjective ending is -e:

die ruhige Stärke (accusative feminine singular)
similar patterns:
- die schöne Wohnung (I see the beautiful apartment. → Ich sehe die schöne Wohnung.)
- die interessante Geschichte (She reads the interesting story. → Sie liest die interessante Geschichte.)

ruhigen or ruhiger would be needed in other case/gender/article combinations, but not in this one.

What does vor allem mean, and where can it go in the sentence?

vor allem means above all, especially, in particular.

In your sentence:

Die Leserin findet vor allem die ruhige Stärke der Hauptfigur beeindruckend.

it highlights what she finds particularly impressive.

Typical positions for vor allem:

After the verb:
- Die Leserin findet vor allem die ruhige Stärke … beeindruckend.
At the beginning (for stronger emphasis):
- Vor allem findet die Leserin die ruhige Stärke der Hauptfigur beeindruckend.

You can also put it later, but then it tends to sound different or a bit awkward:

Die Leserin findet die ruhige Stärke der Hauptfigur vor allem beeindruckend.
→ Emphasizes “impressive above all” (slightly unusual, but possible).

So the most natural here is the original:

… findet vor allem die ruhige Stärke … beeindruckend.

Why is beeindruckend at the end of the sentence?

German main clauses follow the verb-second rule:

The conjugated verb (findet) must be in second position.
Other elements (object, adverbials, predicate) go into the “Mittelfeld” and then the end.

Your sentence is structured like this:

Die Leserin – first position (subject)
findet – second position (finite verb)
vor allem die ruhige Stärke der Hauptfigur – middle field (adverb + object)
beeindruckend – end position (predicate adjective)

The pattern finden + Akkusativobjekt + Adjektiv is very common:

Ich finde den Film langweilig.
Wir fanden das Essen lecker.
Sie findet die ruhige Stärke der Hauptfigur beeindruckend.

In such structures, the adjective that describes the object typically comes at the end.

What is the difference between beeindruckend and beeindruckt?

beeindruckend = impressive (the thing has the quality)
beeindruckt = impressed (the person feels that way)

In your sentence:

die ruhige Stärke der Hauptfigur is beeindruckend
→ The quiet strength is impressive.

If you wanted to say the reader is impressed, you’d use beeindruckt:

Die Leserin ist vor allem von der ruhigen Stärke der Hauptfigur beeindruckt.
→ The reader is impressed above all by the quiet strength of the main character.

So:

X ist beeindruckend. → X is impressive.
Y ist von X beeindruckt. → Y is impressed by X.

Could I rephrase the sentence without finden? For example with ist beeindruckt?

Yes. A very natural alternative is:

Die Leserin ist vor allem von der ruhigen Stärke der Hauptfigur beeindruckt.

Comparison:

Die Leserin findet vor allem die ruhige Stärke der Hauptfigur beeindruckend.
- literally: The reader finds the quiet strength of the main character impressive.
- uses finden + Akkusativ + Adjektiv
- slightly more evaluative / opinion-like.
Die Leserin ist vor allem von der ruhigen Stärke der Hauptfigur beeindruckt.
- literally: The reader is impressed above all by the quiet strength of the main character.
- uses sein + (von …) beeindruckt
- focuses more directly on her emotional reaction.

Both are correct. The original sentence has a bit more of an evaluative judgment feel (finden … beeindruckend), while the rephrased one emphasizes the reader’s emotional state (ist beeindruckt).