Use case writing is key to effective requirements management. Each use case represents a single idea or logically grouped behaviors. When you define a use case, there are several mistakes you can make. Preventing those mistakes is the first order of business. The second order of business is making sure that the use cases in the system work together. This requires an understanding of the context in which the use case happens. To fully understand a use case you have to know what is promised to be true before the use case happens, as well as what causes the use case to happen. These are subtly different.