Having understood what functional test cases are, let's take a look at how they're written. Below is a list of points you should consider while writing test cases: 

• The first step is to have a broad overview of the testing project and determine what should be tested.  
• Now it’s time to write the test cases. Before writing test cases, it is important to understand the client's requirements clearly. According to the REQUIREMENT DOCUMENT specifications, every functional and nonfunctional requirement should be covered, from UI interfaces to compatibility. Normally, a traceability matrix is maintained to ensure that all requirements are implemented and tested.  
• It is a good IDEA to periodically check test cases to ensure that they are not redundant. 
• While writing a test case, priority is an important CONSIDERATION. In this way, the tester is able to begin testing the application with the high priority test cases, followed by the medium and then the low priority test cases.
• Test cases should be written in a simple LANGUAGE and in a structure that is easy to understand. For test cases, the input data values should be valid and within a wide range.