"Dan is het een vulkaan."
Translation:Then it is a volcano.
Basically, to expand on what Susande said, the first verb, the one that is conjugated, should always go in the second place in the sentence, except in questions, when it comes first (except for the exceptions...). Using Susande's examples, you can see in the first sentences that 'is' (conjugated form of 'zijn', to be) is the second word in the sentence. It follows the subject, 'Het' (It). In the second sentence, the verb remains in the second place although the subject (het) is now in third place, having been ousted by 'Dan' (Then). Finally, it is still the second word in the last sentence, in a somewhat complicated way, because the subject, rather than being one word, is three words long: The mighty Vesuvius. It is a fairly strict rule, that the first verb be placed in the second spot in the sentence. The only tricky part is when you find complicated subjects, but just try to remember that the verb will come after the subject (again, unless it is a question, in which case the rule is pretty much the opposite and the verb comes first). If there are any more verbs in the sentence, they all get thrown onto the end of the sentence and don't get conjugated. That is a very, very simplistic explanation and keep in mind that there are always exceptions and many rules and complex sentence structures that make this rule difficult to follow, but it's a basic format.