Test cases using use cases?

Hi, how to write test cases using use cases, explain with any simple example?

Use case is a document which will be prepared from the Functional requirement specification. The document will have the Basic flow, Alternative flow and exceptional flow in which the application flows. Use case document will be used by both the Developers and the Testers as a base document. All the test scenarios will be derived from the Use Case document.

For example in a log in window the basic flow will be getting into the next screen for a valid user name and password. Exceptional flow will be the error message thrown for invalid users and the Alternate flow will be if there is a link for checking the users avaialability in the database.

Based on the above information in the use case of the login window, the test scenarios will be identified for which the test cases are written.