Questions & Answers about Мэр работает в городе.
- мэр is the subject: the person doing the action.
- работает is the verb: it tells you what the subject does.
- в городе is a prepositional phrase: it tells you the location.
So the structure is:
subject + verb + location
This is a very normal, neutral Russian sentence pattern.
Because в городе expresses location: the mayor works in the city.
After в, Russian uses different cases depending on the meaning:
- в городе = in the city / located there
- в город = into the city / movement toward it
So:
- Мэр работает в городе. = location
- Мэр едет в город. = movement
That is why город changes to городе here.