Predicate nominatives

I received this question:

My specific question is from Visual Latin 1, Lesson 25B number 20. In the sentence “Hic homo rex bonus est.”, why is the word for king in the nominative form “rex”? Why is it not the accusative “regem”?

Here is my reply:

The reason rex is in the nominative case is because it’s a predicate nominative.  The following videos may help: