Die Vorladung nennt den Gerichtssaal.

Because Vorladung is a feminine noun in German. In the nominative singular (the form used for the subject of the sentence), the definite article for feminine nouns is die.

So:

• die Vorladung = the summons / the subpoena

Here, Die Vorladung is the subject, the thing doing the action.

Because Gerichtssaal is the direct object of the verb nennt, and masculine nouns take den in the accusative case.

So:

• der Gerichtssaal = nominative masculine
• den Gerichtssaal = accusative masculine

In this sentence:

• Die Vorladung = subject
• nennt = verb
• den Gerichtssaal = direct object

A useful pattern is:

• der → den for masculine nouns in the accusative

Nennt is the 3rd person singular present tense form of the verb nennen.

The subject is Die Vorladung, which is grammatically singular, so German uses the er/sie/es form of the verb:

• ich nenne
• du nennst
• er/sie/es nennt
• wir nennen
• ihr nennt
• sie/Sie nennen

So Die Vorladung nennt ... means The summons states/mentions ...

Not exactly in the everyday sense of to call someone by a name. Nennen can have several related meanings, including:

• to name
• to mention
• to state
• to specify

In this legal/official context, nennt is better understood as states, specifies, or mentions.

So the sentence is not saying the summons is calling the courtroom something; it is saying the summons states which courtroom it is.

This is a very normal German main clause word order:

Subject + Verb + Object

• Die Vorladung = subject
• nennt = verb
• den Gerichtssaal = object

German main clauses usually put the finite verb in second position. Here, the subject happens to come first, so the result looks very similar to English word order.

Because all nouns are capitalized in German.

So both of these words are nouns:

• Vorladung
• Gerichtssaal

This is a standard rule in German spelling, not something special about this sentence.

Yes. German very commonly forms compound nouns by joining words together.

Gerichtssaal is made up of:

• Gericht = court
• Saal = hall / chamber / room

Together, Gerichtssaal means courtroom.

This is extremely common in German, and English speakers often need time to get used to long single-word compounds.

The noun is masculine: der Gerichtssaal.

One clue is the final element of the compound. In German compounds, the last part usually determines the gender. Since der Saal is masculine, der Gerichtssaal is also masculine.

So:

• der Saal
• der Gerichtssaal

Then in the accusative it becomes:

• den Gerichtssaal

Yes, depending on context.

• Die Vorladung = the summons (a specific one)
• Eine Vorladung = a summons (non-specific)

So the grammar of the rest of the sentence would stay basically the same:

• Eine Vorladung nennt den Gerichtssaal.

The choice between die and eine depends on whether the speaker means a particular summons already known in context.

The articles help you.

• Die Vorladung is nominative feminine singular, so it is the subject
• den Gerichtssaal is accusative masculine singular, so it is the direct object

German case marking often tells you the role of each noun in the sentence, even if the word order changes.

So in this sentence, the articles make the structure clear.

A rough English-friendly guide is:

• Vorladung ≈ FOR-lah-doong
• Gerichtssaal ≈ geh-RIKHT-zahl

A few pronunciation notes:

• v in many German words is pronounced like f, so Vor- sounds like for
• ch in Gericht is the German sound made in the back of the mouth, not like English ch in chair
• aa in Saal is a long a sound

The stress is usually:

• VORladung
• GeRICHTSsaal