How do I know what content will appear on my HAT question 2 when I'm not studying the same periods as anybody else?

The university specifically designs the HAT to ensure that anybody who does some form of history can answer it. Section two on the paper will be deliberately open, so you don't have to worry about knowing a particular period of history - say medieval England, or Apartheid South Africa. If you look at the practice question for the 2017 paper, for example, it says:"With regard to any historical event or process of change with which you are familiar, write an essay answering the following question: Did change come primarily from the top, from the bottom, or from any other level of society? "As you can see, the phrases in bold are deliberately open-ended. They don't mention an event, like the beginning of Apartheid, or a person, like Nelson Mandela, or a period, like the 20th century. The key is that YOU decide what you want to talk about - which is exciting! Therefore, the best way to prepare is find an area of history that interests YOU that you can get really comfortable with and use flexibly to respond to any question that comes up. Or, to be doubly sure, you can keep a couple of events/ periods/ eras in mind that you could apply to different situations.