How do I write a good introduction for an essay?

When writing an introduction it is a good idea to stick to a clear three-part structure. Let's take the following example of an essay question: How did the role of women in Britain change during World War II?
The first part of the introduction is 'context'. So explain WWII; when it happened, who was involved, why the role of women changed in this period. (write 2-3 sentences)
The second part of the introduction is 'definition'. This involves defining any keywords in the question. The question is quite broad, but it might be useful to explain that by 'role' we mean political, economic and social positions. We might also define the role of women before the war; laying the foundations for the argument which we will explain next. (write 1-2 sentences)
The third part of the introduction is 'argument'. This involves setting up the main body of the essay. Setting up an argument in the introduction will ensure clarity for the reader, ad it will help you to maintain a clear structure over the course of your essay. This will be especially useful in an exam. (write roughly 3-4 sentences)
Depending on the length of the essay, we might want to break our argument into three points (three paragraphs in the essay). The first point may be that more women went to work in factories (economic). The second point might be that women were less involved with childcare due to evacuation (social). The last point might be that the changing role of women social and economically led to women being more involved politically (political).
If you follow this general 3 part structure, your introduction will be clear and concise; helping you to write a clear and concise essay.