Learn Dutch: Als iemand gewond is, dan belt u de ambulance. - If someone is injured, then you call the ambulance.

The word dan (then) acts as a resumptive adverb here. It refers back to the entire condition you just set up in the first clause (Als iemand gewond is), making the cause-and-effect relationship much clearer. It isn't strictly required—you could grammatically say Als iemand gewond is, belt u de ambulance—but adding dan is extremely common and sounds very natural in Dutch.

Could I use zo instead of dan in this sentence?

No, because this is a conditional sentence. We use dan for logical consequences, which perfectly matches an als (if) clause. You would use zo to resume a manner or method, meaning "in this way" or "like this" (for example, if the first clause was about how to perform first aid, rather than whether someone needs it).

Why is the word order dan belt u instead of dan u belt?

In a main Dutch clause, the verb must be in the second position (the V2 rule). The entire first clause plus dan acts as the first block of information. Because that takes up position one, the verb belt must come immediately next in position two, forcing the subject u to invert and come after it.

Why does the first part of the sentence end with the verb is?