How do I structure a 'discuss' exam answer?

You should structure this as a fairly balanced answer. You want to involve more than one argument (preferably more than two), but argue both equally. This will show the examiners you understand the topic broadly, as well as the different pieces of historiography. For example, if you had 'Discuss the causes for one 20th century civil war', you could try to argue economic , political, and social sides. You would have a brief introduction of the different aspects, three equally argued paragraphs, and a closing that makes reference to what was perhaps the most important cause. This format allows you to be concise (helpful in a timed essay form exam) as well as adding in lots of information that will help boost your knowledge and understanding criterion.