A singular subject needs a singular verb:
  The king sings when he is happy.

A plural subject needs a plural verb:
  The kings sing when they are happy.

Don’t be confused by intervening words:
  The king, living in that big castle, sings when he is happy.
  The king, along with the queen, sings when he is happy.
  The kings, living in those big castles, sing when they are happy.

Don’t be confused by an intervening prepositional phrases:
  The king of England sings when he is happy.
  All kings of England sing when they are happy.