If it's a good textbook, you can usually count on the events described being based on careful, high-quality research by historians.

But the best research rarely yields 100% certainty. Even today, when many eyewitnesses watch events happen, we often struggle to determine the real details of important events (look, for example, at the arguments about the shooting of Michael Brown, and the vast differences between the eyewitness accounts that are recorded in the court documents).

When we study events in the distant past, our information is often much more limited than for contemporary events, so a degree of uncertainty is always assumed. Professional historians' job is to uncover as much evidence as possible, weigh it carefully, and explain why we think it should be understood in a particular way.

Some things in the past are pretty certain - we have a lot of evidence about the Holocaust, and agree on what it means on all the major points. Some things have been debated intensely, and it's still not clear what version of events is best (for example, why did the Roman emperor Constantine convert to Christianity? There are many theories!). Some things have so little evidence surviving that we're not sure we know what happened, and we don't have enough evidence to learn more.

A good textbook will tell you about the thorniest debates, and give you a few of the most widely accepted theories, along with a bit of the reasoning behind them so you can try to make sense of them for yourself. But whether a textbook tells you something is controversial or not, you should always be aware that there's a pile of research, evidence, and interpretation behind every 'fact' the book reports - you could, if you wanted, question and challenge anything in your textbook, and you'd almost always learn that the real story behind the facts it reports is more complicated (and usually, more interesting!).

Professional historians spend our careers digging through the evidence to make sense of this complicated background (and if you go on to study to become a professional historian, this - the ability to sort through and evaluate complicated evidence - is the skill you'd be trained to master).