Writing an essay for history is not necessarily the same as it may be for an English class. It is not enough to describe what happened or to write a narrative of past events.
Here you give your carefully thought out definitions of the key terms, and here you establish the relevant time-frame and issues — in other words, the parameters of the question.
More important is that you demonstrate your understanding of the question set. By all means have a dramatic first sentence — to shock the reader from the stupor that prolonged marking invariably induces — but do not merely 'set the scene' or begin to 'tell a story'. Every argument should be accompanied with evidence.
Each middle paragraph should have an argument or interpretation or generalisation supported by evidence. In other words, you have to think very carefully about the question you are asked to answer.
The introduction — the introduction is where you start your writing. It is better that you have simple, language to communicate your argument clearly than have unnecessary words that doesn't make sense. Thinking is rarely a pleasant undertaking, and most of us contrive to avoid it most of the time.
Unless you give real evidence to back up your view — as historians do — a generalisation is simply an assertion. For example, the question can be: 'Examine the role of women in World War One? For term-time essays, presentation is important. You have to think and think hard — and then you should think again, trying to find loopholes in your reasoning.