Learn Dutch: Zij vragen elkaar waar de dader nu is. - They ask each other where the perpetrator is now.

The second half of the sentence (waar de dader nu is) is an indirect question. Indirect questions use the same word order as subordinate clauses, meaning the verb gets pushed all the way to the end.

How would the word order change if we turned the second half into a regular, direct question?

If you asked it directly, the verb would come right after the question word: Waar is de dader nu? (Where is the perpetrator now?). Because it is an indirect question here, the verb is has to wait until after the subject and the time word.

What does elkaar mean in this sentence?

Elkaar translates to 'each other'. It is used when an action goes both ways between people, so zij vragen elkaar simply means 'they ask each other'.

Is there a trick to remembering the word dader (perpetrator)?

Yes! Dader comes from the noun daad (deed), which is related to the verb doen (to do). You can think of a dader literally as the 'doer' of a crime.