Die Polizei fragt nach dem Aktenzeichen.

Because nachfragen nach + dative is the standard way to say to ask for / to inquire about something in German.

• fragen on its own often takes a direct object or a person:
  • Die Polizei fragt den Zeugen. = The police question the witness.
• fragen nach + dative means asking about a specific thing:
  • Die Polizei fragt nach dem Aktenzeichen. = The police are asking for/about the file number.

So here, nach is required by the verb pattern.

Because the preposition nach takes the dative case.

• Nominative: das Aktenzeichen
• Dative: dem Aktenzeichen

So:

• nach das Aktenzeichen ❌
• nach dem Aktenzeichen ✅

This is one of the most important things to memorize in German: many prepositions always require a specific case, and nach always takes dative.

Aktenzeichen is an official reference number used for a case, file, or document, especially in legal, police, or administrative contexts.

You can think of it as something like:

• case number
• file number
• reference number

It is a very bureaucratic or official word, so it often appears in letters, court matters, police reports, and government paperwork.

Grammatically, die Polizei is singular in German, even though in English the police is often treated as plural.

That is why the verb is:

• Die Polizei fragt ✅
• Die Polizei fragen ❌

German treats die Polizei as a singular collective noun, similar to the police force.

Yes, the word order can change, but the conjugated verb still stays in the second position in a normal main clause.

Standard order:

• Die Polizei fragt nach dem Aktenzeichen.

Other possible orders:

• Nach dem Aktenzeichen fragt die Polizei.
• Heute fragt die Polizei nach dem Aktenzeichen.

In each case, the conjugated verb fragt remains in second position. German is a verb-second language in main clauses.

The dictionary form is fragen.

Here, fragt is the third-person singular present tense form:

• ich frage
• du fragst
• er/sie/es fragt
• wir fragen
• ihr fragt
• sie/Sie fragen

Since die Polizei is singular, German uses fragt.

Yes. Depending on context, nach etwas fragen can mean:

• ask for something
• ask about something
• inquire after something

So in this sentence, it could mean that the police want someone to provide the file number, or that they are asking about which file number is relevant. The exact shade of meaning depends on the situation.

There actually is one: dem already includes the meaning of the.

German articles change depending on case, gender, and number. So:

• das Aktenzeichen = the file number
• dem Aktenzeichen = to/about/for the file number, after a dative preposition like nach

So dem is not just a case ending added to the noun—it is the full article.

It is neuter:

• nominative: das Aktenzeichen
• accusative: das Aktenzeichen
• dative: dem Aktenzeichen
• genitive: des Aktenzeichens

A useful clue is that many nouns ending in -zeichen are neuter, because Zeichen itself is neuter: das Zeichen.

Because many neuter nouns do not change in the singular dative. The article changes instead:

• das Aktenzeichen → dem Aktenzeichen

So the important visible change here is mostly in the article, not the noun itself.

Only some nouns show additional noun-ending changes in certain cases, but Aktenzeichen usually stays the same in nominative, accusative, and dative singular.

Not naturally in the same way. Die Polizei will das Aktenzeichen literally means The police want the file number, which is grammatical but much less specific.

fragt nach dem Aktenzeichen tells you that the police are actively asking for or inquiring about it. It describes the action more precisely.

So:

• Die Polizei fragt nach dem Aktenzeichen. = The police are asking for/about the file number.
• Die Polizei will das Aktenzeichen. = The police want the file number.

Both are possible, but they do not mean exactly the same thing.

It can be understood in two closely related ways:

1. fragen nach + dative = to ask for/about something
2. nachfragen = to inquire / ask again / follow up

In this sentence, the most direct analysis is fragen nach with the preposition nach:

• Die Polizei fragt nach dem Aktenzeichen.

That is the normal pattern learners should remember.
You may also see nachfragen as a dictionary entry, but in sentences like this, it is best to learn that nach belongs with the meaning and requires dative.