Die Polizei fragt das Opfer nach dem Tatort.

In German, die Polizei is grammatically singular, even though in English the police is usually treated as plural.

So German says:

• Die Polizei fragt ... = The police ask/is asking ...

That is why the verb is fragt (3rd person singular), not fragen.

You can think of die Polizei as a collective noun here, more like the police force.

Opfer is a neuter noun, so its basic form is:

• das Opfer = the victim

In this sentence, it is the direct object of fragt, so it is in the accusative case. For neuter nouns, the article das stays the same in nominative and accusative:

• nominative: das Opfer
• accusative: das Opfer

So das Opfer is correct here.

Because of the preposition nach.

In this sentence, German uses the pattern:

• jemanden nach etwas fragen = to ask someone about something

Here:

• jemanden → the person being asked → das Opfer
• nach etwas → the topic being asked about → dem Tatort

Since nach always takes the dative, der Tatort becomes dem Tatort.

This is just the normal German construction with fragen when you ask someone about something:

• jemanden fragen = ask someone
• jemanden nach etwas fragen = ask someone about something

So:

• Die Polizei fragt das Opfer nach dem Tatort.

Literally, that is something like:

• The police ask the victim concerning/about the crime scene.

English uses about, but German commonly uses nach in this pattern.

The basic verb is still fragen.

You can use it in different ways:

• jemanden fragen = to ask someone
• nach etwas fragen = to ask about something
• jemanden nach etwas fragen = to ask someone about something

So in this sentence, both parts appear together:

• die Polizei = subject
• fragt = verb
• das Opfer = person being asked
• nach dem Tatort = topic being asked about

This is normal German main-clause word order:

• Subject: Die Polizei
• Verb: fragt
• then the other elements: das Opfer nach dem Tatort

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

So this is the standard pattern:

• Die Polizei | fragt | das Opfer nach dem Tatort

You could move other parts to the front for emphasis, but the verb would still stay in second position:

• Nach dem Tatort fragt die Polizei das Opfer.

That sounds more marked or emphatic.

German very often forms compound nouns, where two or more words are joined together into one.

Here:

• Tat = deed, act, crime
• Ort = place

So:

• Tatort = crime scene / literally crime-place

This is completely normal in German. English often writes similar ideas as separate words, but German usually combines them.

Because they are nouns, and in German all nouns are capitalized.

So:

• die Polizei
• das Opfer
• der Tatort

This is one of the most noticeable spelling differences between German and English.

Yes.

German present tense often covers both:

• The police ask the victim about the crime scene
• The police are asking the victim about the crime scene

German does not usually need a separate progressive form like English is asking / are asking.

So fragt can refer to a general present or an action happening now, depending on context.

Yes. Opfer is grammatically neuter no matter who the victim is.

That is because grammatical gender in German does not always match natural gender.

So you can have:

• das Opfer war ein Mann
• das Opfer war eine Frau

The noun Opfer stays neuter either way.

Sometimes German uses different prepositions depending on the verb and style, but with fragen, the very common standard pattern is:

• jemanden nach etwas fragen

So nach dem Tatort is the natural choice here.

A phrase with über would usually fit better with other verbs, for example:

• mit jemandem über den Tatort sprechen = to speak with someone about the crime scene

So in this sentence, nach dem Tatort fragen is the idiomatic form.

Here is the full breakdown:

• Die Polizei → nominative
  It is the subject.

• das Opfer → accusative
  It is the direct object, the person being asked.

• dem Tatort → dative
  It comes after nach, which requires dative.

So the sentence shows three different grammatical roles very clearly:

• who is doing the asking
• who is being asked
• what they are being asked about