IN: Which person is he?
Steve Ballmer took over as CEO of Microsoft after Bill Gates because he retired.
OUT: Bill Gates

IN: Which person is they?
Walmart has leaked pages of their Black Friday Ad to the users of the internet, but they will have many other ads to look at to choose from.
OUT: the users of the internet

IN: Which person is they?
Frosted Flakes is tastier than Fiber One because they taste bland due to the fiber.
OUT: Fiber One

IN: Which person is he?
Chris was running after John, because he wanted to talk to him.
OUT:
Chris