As a pronoun they usually talks about a group of people, but it can also talk about one person.
Does this sound good?
- I don’t know who broke the window, but he or she is going to pay for a new one!
Of course it doesn’t. So, we use they.
- I don’t know who broke the window, but they are going to pay for a new one!
- I don’t know who was driving the bus, but they were very good.
- Whoever cleans the kitchen, I wish they would do a better job.
