Morgen sagt der Zeuge auf der Polizeiwache aus.

Because the verb is aussagen, which is a separable verb.

In a main clause, German usually puts the conjugated part of the verb in second position and the separable prefix at the end:

• Der Zeuge sagt aus.
• Morgen sagt der Zeuge auf der Polizeiwache aus.

In the dictionary, the verb is written together as aussagen. In this sentence, it splits into sagt ... aus.

Not quite. Aussagen often means to testify, to give a statement, or to make an official statement.

So it is more specific than ordinary sagen. Because the sentence has der Zeuge and auf der Polizeiwache, the meaning is clearly something like giving testimony or making a statement to the police.

German main clauses follow the verb-second rule. That means the finite verb must come in the second position.

If you put Morgen first for emphasis, the verb still has to stay second, so the subject comes after it:

• Morgen sagt der Zeuge ...
• more neutral: Der Zeuge sagt morgen ...

This is very normal in German. English learners often expect the subject to come first, but German is more flexible.

Here it means tomorrow.

A few useful points:

• morgen as an adverb = tomorrow
• der Morgen as a noun = the morning

At the beginning of a sentence, morgen is written with a capital letter anyway, so capitalization alone does not help you. You understand it from context.

If German wanted to mean in the morning, it would more naturally say something like:

• morgen früh
• am Morgen

So in this sentence, Morgen is understood as tomorrow.

Because German very often uses the present tense to talk about the future when the time is already clear.

Here, Morgen makes the future meaning obvious, so sagt ... aus is completely natural.

German could also say:

• Morgen wird der Zeuge auf der Polizeiwache aussagen.

But that is not necessary. The simple present is very common and often sounds more natural.

Der Zeuge is nominative, because it is the subject of the sentence: the witness is the person doing the action.

The verb is sagt ... aus, and the witness is the one making the statement, so der Zeuge must be the subject.

Also, the article der fits masculine nominative singular here.

Because auf is a two-way preposition:

• dative for location
• accusative for direction/movement toward a destination

In this sentence, the witness is giving a statement at the police station, so it is a location. That is why German uses the dative:

• auf der Polizeiwache

Compare:

• Er ist auf der Polizeiwache. = He is at the police station.
• Er geht auf die Polizeiwache. = He goes to the police station.

Because Polizeiwache is a feminine noun: die Polizeiwache.

After auf with a location meaning, German needs the dative, and the feminine dative singular article is der:

• nominative: die Polizeiwache
• dative: der Polizeiwache

So auf der Polizeiwache means at the police station.

Yes, absolutely. That is also correct.

German word order is flexible as long as the main rules are followed, especially:

• the finite verb stays in second position
• the separable prefix aus stays at the end in a main clause

So both of these are correct:

• Morgen sagt der Zeuge auf der Polizeiwache aus.
• Der Zeuge sagt morgen auf der Polizeiwache aus.

The difference is mainly emphasis. Starting with Morgen highlights the time. Starting with Der Zeuge is more neutral.