Learn German: Sie müssen die Lizenz kaufen, andernfalls untersagen wir die Nutzung. - You must buy the license, otherwise we will prohibit its use.

Because andernfalls is an adverbial connector, it takes up the first position in its clause. Just like with any regular adverb at the start of a sentence, the verb (untersagen) must come second, pushing the subject (wir) to the third position. This makes it different from coordinating conjunctions like und or oder, which take "position 0" and leave the subject in position 1.

Is andernfalls used exactly like sonst ("otherwise")?

Yes, they mean the same thing and work exactly the same way grammatically. However, andernfalls is much more formal. It literally translates to "in another case" (anderer + Fall + adverbial s), which highlights its conditional nature: if the first condition isn't met, the second thing happens. It perfectly matches the formal, legal tone of this sentence.

Could I put andernfalls somewhere else in the second clause?

Yes! Because it functions as an adverb, you don't have to put it at the very beginning of the clause. You could also start with the subject and say: ... wir untersagen andernfalls die Nutzung.

I usually use verbieten for "to forbid" or "to prohibit". Why use untersagen here?