Das Opfer beschreibt den Täter in der Zeugenaussage.

Because the two nouns are in different cases and have different grammatical genders.

• das Opfer = the subject of the sentence, so it is in the nominative
• den Täter = the direct object, so it is in the accusative

Also:

• Opfer is a neuter noun, so its article in the nominative is das
• Täter is a masculine noun, so its article changes:
  • nominative: der Täter
  • accusative: den Täter

So the sentence structure is:

• Das Opfer = the victim
• beschreibt = describes
• den Täter = the perpetrator/offender

Because grammatical gender in German does not always match natural gender.

Das Opfer is always grammatically neuter, even when talking about a male or female victim. This is normal in German. A noun’s article depends on the noun itself, not necessarily on the real-life sex of the person.

So you say:

• das Opfer = the victim

But later, German may use pronouns based either on grammatical gender or on the person’s natural gender, depending on context and style.

Yes. beschreiben is a verb that normally takes a direct object in the accusative.

Examples:

• Sie beschreibt den Mann.
• Er beschreibt die Situation.
• Das Opfer beschreibt den Täter.

So in your sentence, den Täter is accusative because it is the thing/person being described.

It means something like in the witness statement or in the testimony.

Here, in is being used in a more abstract sense: within the statement/testimony.

Zeugenaussage is a compound noun:

• der Zeuge / die Zeugin = witness
• die Aussage = statement, testimony

Together:

• die Zeugenaussage = witness statement / testimony

So in der Zeugenaussage means the description happens as part of the witness statement.

Because in can take either the dative or the accusative, depending on meaning.

• dative = location, position, being in something
• accusative = movement into something

Here there is no movement. The meaning is within the witness statement, so German uses the dative:

• nominative: die Zeugenaussage
• dative: der Zeugenaussage

So:

• in der Zeugenaussage = in the witness statement

If it were movement, it would be accusative, but that would not fit this sentence.

Yes, it is very normal.

The sentence follows the common German main-clause pattern:

Subject + verb + object + additional information

So:

• Das Opfer = subject
• beschreibt = verb
• den Täter = object
• in der Zeugenaussage = prepositional phrase

German main clauses usually put the finite verb in second position, and that is exactly what happens here.

Yes. German is flexible, as long as the finite verb stays in second position in a main clause.

For example:

• Das Opfer beschreibt den Täter in der Zeugenaussage.
• In der Zeugenaussage beschreibt das Opfer den Täter.

Both are grammatical.

The second version puts more emphasis on in the witness statement. English is less flexible here, but German often moves sentence elements for emphasis or style.

This is an important vocabulary point.

• der Täter = the perpetrator / offender / culprit
  This suggests the person did the act.
• der Verdächtige = the suspect
  This means the person is only suspected.

So den Täter is stronger and more definite than den Verdächtigen.

Depending on legal or factual context, German may choose one very carefully.

Yes. German very often forms compound nouns by writing them as a single word.

So:

• Zeuge
  • Aussage → Zeugenaussage

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

Other similar examples:

• Polizeibericht = police report
• Gerichtssaal = courtroom
• Augenzeuge = eyewitness

It is present tense.

• beschreiben = to describe
• er/sie/es beschreibt = he/she/it describes

So Das Opfer beschreibt den Täter literally means The victim describes the perpetrator.

In context, German present tense can sometimes also be translated naturally into English in other ways depending on the situation, but the form itself is simple present.

Because many German nouns do not change their form across cases, especially in the singular. The article often shows the case more clearly than the noun itself.

Here:

• nominative: das Opfer
• accusative: das Opfer

The noun Opfer stays the same. The article das also happens to stay the same because for neuter singular, nominative and accusative are identical.

That is why you have to pay attention to:

• the article
• the verb
• the sentence role

to understand who is doing what.