First discussion about test case.When tester get task to create test cases then a report format comes in mind but first of all conditions should come according to provided requirements what is needed to test then tester should think about how to test with some scenarios so it means when a complete step is created to test a task then it is a test case.
Like a task is verified with created steps then it is test case. For more reference, a task is assigned related to search functionality then just need to create positive conditions and negative conditions and those created conditions will be called as test cases.
With practical knowledge,
1 Test Case: Enter positive data and click on search button. It means this a test case that will be executed further in testing time.
2 Test Case: Enter blank data and click on search button. It means this a negative test case that will be executed further in testing time.
Now it is about use case. Main thing is about use of use case to identify the test case that helps to create test case exactly according to requirement. Mainly Business Analyst or a Senior person will be involved to identify the business requirements with proper description then use cases will be shared to testing team to understand exact requirement and further team create test cases with help of use cases.
With a simple example: Like a requirement is written related to sign up form and it is mentioned to implement with phone number and email id. So it means use case will be helpful for a tester to write positive and negative test cases related to phone number and email id.