Learn Dutch: We verstevigen het plafond, zodat het niet instort. - We are reinforcing the ceiling, so that it does not collapse.

It transforms the adjective stevig (firm, sturdy) into a verb of state change. By adding the prefix ver- and the infinitive ending -en, you get verstevigen, which literally means "to make firm" or "to reinforce."

Why is the separable verb instorten written together as instort at the end of the sentence?

Zodat is a subordinating conjunction, which means it forces the conjugated verb to the very end of the clause. When a separable verb like instorten (to collapse) is pushed to the end of a subordinate clause, its prefix (in-) and its verb stem (stort) are forced back together into a single word: instort.

Why do we use het in the second clause instead of hij or die?

The pronoun refers back to the ceiling (het plafond). Because plafond is a neuter noun (a het-word), you must use the pronoun het ("it") to refer back to it. If it had been a de-word (like de muur), you would use hij or die.

Could we use an "om ... te" (in order to) structure here instead of zodat?