The nominative case is used for the grammatical subject of the sentence.
The subject is, the person/thing doing an active verb:
The boy is walking.
Or suffering a passive verb:
rex ab omnibus civibus laudatur.
The king is praised by all the citizens.
A nominative may also be used to complete the verb esse (“to be”). This is called the complement.
Tiberius est princeps.
Tiberius is the emperor.
The complement also completes the passive of facio, “I make, do:”
mox Numa rex factus est.
Soon Numa became king.