I know that
der Bus = the bus
and in the question
Wissen sie, wann der Bus kommt?
I would have thought that seeing the verb "kommen" is being used, the article of the noun "Bus" would be "den" i.e. in the accusative.
The idea I had was:
Wissen sie, wann den Bus kommt?
Why is the initial question in the nominative and not in the accusative?