Learn Dutch: Wanneer de brandweer aankomt, dan volgt u de instructie. - When the fire brigade arrives, then you follow the instruction.

The word dan (then) acts as a resumptive adverb. It summarizes and points back to the entire first clause (Wanneer de brandweer aankomt). It acts as a bridge, emphasizing that the second action happens as a direct sequence to the first.

Is it mandatory to use dan here?

No, you could drop it and just say Wanneer de brandweer aankomt, volgt u de instructie. However, adding a resumptive adverb like dan is very common in Dutch to make the transition between the two clauses smoother and the structure crystal clear.

Why is the word order volgt u instead of u volgt?

Because the main clause starts with the adverb dan, it triggers inversion. In Dutch main clauses, the conjugated verb must be the second element. Since dan takes the first spot, the verb volgt must come right after it, pushing the subject u to the third spot.

Why isn't the verb aankomen split up here?

Aankomen (to arrive) is a separable verb, so in a normal main clause it splits (for example: De brandweer komt aan). But the first part of this sentence is a dependent clause because it starts with Wanneer (when). In a dependent clause, all verbs get pushed to the end and reconnect into one word: aankomt.