Die Spielanleitung erklärt die Siegbedingung.

Because both nouns are feminine singular.

In German, the feminine singular definite article is die in both the nominative and the accusative. So in this sentence:

• die Spielanleitung = the subject
• die Siegbedingung = the direct object

Even though they do different jobs in the sentence, the article looks the same.

Mostly from word order and context.

In a normal German main clause, the finite verb is in second position, and the noun phrase before it is usually the subject in a neutral sentence:

• Die Spielanleitung | erklärt | die Siegbedingung

So the natural reading is that die Spielanleitung is the thing doing the explaining.

This sentence is a good example of something important: for feminine singular nouns, nominative and accusative often look identical, so the article alone does not always tell you the role.

Because erklärt is the conjugated form, while erklären is the infinitive.

The subject is die Spielanleitung, which is third-person singular, so the verb must match it:

• ich erkläre
• du erklärst
• er/sie/es erklärt
• wir erklären
• ihr erklärt
• sie/Sie erklären

So die Spielanleitung erklärt = the game instructions explain / the instruction manual explains, depending on context.

It is accusative.

It is the direct object of erklären—the thing being explained. In other words, the manual explains what?
Answer: die Siegbedingung.

A useful pattern is:

• jemand erklärt etwas = someone explains something
• jemand erklärt jemandem etwas = someone explains something to someone

For example:

• Die Spielanleitung erklärt dem Spieler die Siegbedingung.

Here, dem Spieler would be dative, and die Siegbedingung would still be accusative.

Yes. Both are classic German compound nouns.

They are built like this:

• Spiel
  • Anleitung → Spielanleitung
• Sieg
  • Bedingung → Siegbedingung

German often combines ideas into one long noun where English uses separate words, such as game instructions or victory condition.

The last part of the compound determines the gender.

So:

• die Anleitung → die Spielanleitung
• die Bedingung → die Siegbedingung

That means both full compound nouns are feminine because Anleitung and Bedingung are feminine.

This is a very useful rule for German compounds in general.

In general, yes—die can also be the plural definite article.

But not in this sentence.

You can tell from the verb:

• singular: die Spielanleitung erklärt
• plural: die Spielanleitungen erklären

Since the sentence uses erklärt, the subject must be singular.

Because all German nouns are capitalized.

This is a standard spelling rule in German, not something special about these two words. So nouns like Spiel, Sieg, Anleitung, and Bedingung are capitalized whenever they are used as nouns.

Yes, this is the most neutral German main-clause order:

• subject + verb + object

So:

• Die Spielanleitung erklärt die Siegbedingung.

German word order can change for emphasis, but the finite verb still has to stay in second position in a normal main clause.

In this specific sentence, changing the order can make things less clear, because both noun phrases begin with die and look the same in nominative and accusative. So the standard order is especially helpful here.