Die Polizei nennt mir das Aktenzeichen.

Die Polizei is singular here.

Even though in English the police often behaves like a plural-looking noun, German die Polizei is usually treated as a singular collective noun meaning the police force or the police as an institution.

That is why the verb is nennt, not nennen:

• Die Polizei nennt ... = The police tell/name ...
• not Die Polizei nennen ...

The article die here does not mean the noun is plural. It is simply the feminine singular article for Polizei.

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

The verb nennen often means:

• to name
• to call
• to state/give

In this sentence, nennt is being used in the sense of gives/tells me a specific designation or reference number.

So Die Polizei nennt mir das Aktenzeichen means something like:

• The police give me the file number
• The police tell me the case reference number

Because mir is the dative form of ich, while mich is the accusative form.

The verb pattern here is:

• jemand nennt jemandem etwas

That means:

• someone gives/tells someone something

So the roles are:

• Die Polizei = subject, nominative
• mir = indirect object, dative, the person receiving the information
• das Aktenzeichen = direct object, accusative, the thing being given/told

So:

• mir = to me
• not mich = me as a direct object

Because it is the direct object of the verb.

In this sentence, das Aktenzeichen is the thing that is being named/given/told.

German often works like this:

• subject does the action
• dative object receives something
• accusative object is the thing involved

So in:

• Die Polizei nennt mir das Aktenzeichen

the police tell me the file number.

That makes:

• mir = dative
• das Aktenzeichen = accusative

Also, in this case, das looks the same in nominative and accusative, so the form itself does not change.

Aktenzeichen is a common official or legal term. It means a file reference, case number, or reference number used by authorities, courts, police, and similar institutions.

It is a neuter noun:

• das Aktenzeichen

You will often see it in formal contexts such as:

• police reports
• court documents
• administrative letters
• insurance claims

So this sentence sounds fairly official or bureaucratic.

This is a normal German main-clause word order:

• Die Polizei = position 1
• nennt = position 2
• mir = after the verb
• das Aktenzeichen = later in the clause

German main clauses usually put the conjugated verb in second position.

So the structure is:

• Subject + verb + other elements

Here:

• Die Polizei = first position
• nennt = second position

The sentence is very standard and neutral.

You could also rearrange parts for emphasis, for example:

• Mir nennt die Polizei das Aktenzeichen.

This is grammatically possible, but much less neutral and would sound marked or literary in many contexts.

Nennen is grammatical here, but depending on context, other verbs may sound more natural in everyday German.

Possible alternatives include:

• Die Polizei sagt mir das Aktenzeichen.
• Die Polizei teilt mir das Aktenzeichen mit.
• Die Polizei gibt mir das Aktenzeichen.

Differences:

• sagen = to say/tell; common and straightforward
• mitteilen = to inform; more formal
• geben = to give; very natural in many situations
• nennen = to name/state; often used when specifying a term, number, title, or label

So nennen works especially well because an Aktenzeichen is a fixed official designation.

Yes, you can, if the context already makes clear who receives the information.

• Die Polizei nennt das Aktenzeichen.

This would mean:

• The police state the case number
• The police mention the file number

But with mir, the sentence specifically says that I am the person being told:

• Die Polizei nennt mir das Aktenzeichen = The police tell me the case number

So mir adds the recipient.

Because in German, all nouns are capitalized.

So:

• die Polizei
• das Aktenzeichen

Both Polizei and Aktenzeichen are nouns, so they begin with capital letters.

This is one of the most noticeable differences from English.

Yes, nennt is present tense.

So the basic meaning is:

• The police tell me the case number
• The police are telling me the case number

Like English, German present tense can also sometimes refer to the near future if the context makes that clear:

• Die Polizei nennt mir das Aktenzeichen morgen.
• The police will tell me the case number tomorrow.

So German often uses the present tense where English might prefer will.

The dictionary form is nennen.

A useful pattern to learn is:

• jemand nennt jemandem etwas

Example:

• Die Polizei nennt mir das Aktenzeichen.

You may also meet another common pattern:

• jemanden etwas nennen
• to call someone/something something

For example:

• Er nennt mich einen Freund.
• He calls me a friend.

So nennen can behave in different ways depending on meaning. In your sentence, it works with:

• a dative person: mir
• an accusative thing: das Aktenzeichen

That is the important pattern to remember here.