Historians ask many kinds of questions that help them try to understand the past. Let us look at three main types of questions that they ask.
First, historians ask what happened in the past. In other words, they are asking about basic facts. They need to know things like what empire ruled a given area at a given time. By asking these questions, they get a general understanding of the facts about a given time and place.
Second, historians ask why things happened the way they did. Historians are not content to know what happened at a given time and place. This is not nearly as interesting (in most cases) as trying to find out why things happened. For example, it is more interesting to try to determine why the ancient kingdoms of the Late Bronze Age in the Middle East fell apart than it is to simply know that they did. (This link, for example, shows how historians think they have found evidence about why those kingdoms fell.)
Finally, historians ask how they can know what happened and why it happened. In other words, historians ask about the quality and the nature of the evidence that is available. Historians need to make sure that they are basing their conclusions on solid evidence, not simply on conjecture. Therefore, they have to question their evidence to be sure that it really tells them what they think it does.
Thus, historians try to understand the past by asking questions such as “what happened,” “why did it happen,” and “how do we know these things?”