Learn Dutch: Nergens hang je zo lang ondersteboven als hier. - Nowhere do you hang upside down for as long as here.

Placing the negative adverb Nergens (Nowhere) at the very beginning of the sentence for emphasis triggers inversion. Because of the strict Dutch "V2 rule", the conjugated verb (hang) must remain in the second position, which pushes the subject (je) right behind it.

Since the subject is je (you), shouldn't the verb be spelled hangt?

This is a quick refresher of an old spelling rule that is easy to forget when dealing with complex inversions: whenever the singular informal subject je or jij directly follows the verb, the -t ending is dropped. In standard order it is Je hangt nergens..., but inverted it becomes Nergens hang je...

Is ondersteboven a separable prefix here?

No, ondersteboven (upside down) is functioning as a standalone adverb here describing how you hang, not as part of a compound verb like onderstebovenhangen. It literally breaks down to "under-side-above" (bottom-up).

Why do we use als hier instead of dan hier for the comparison?