Every tester writes test cases however many times the test cases are rejected by reviewers because of bad quality, in order to write good test cases one should know what are the characteristics of a good test case.
A good test case has certain characteristics which are:
1. Should be accurate and tests what it is intended to test.
2. No unnecessary steps should be included in it.
3. It should be reusable.
4. It should be traceable to requirements.
5. It should be compliant to regulations.
6. It should be independent i.e. You should be able to execute it in any order without any dependency on other test cases.
7. It should be simple and clear, any tester should be able to understand it by reading once.
8. Now keeping in mind these characteristics you can write good and effective test cases.
TIPS for writing good test cases
Tests only one thing
Always make sure that your test case tests only one thing, if you try to test multiple conditions in one test case it becomes very difficult to track results and errors.
Organize your test cases consistently
You can organize your test cases in many ways however you should always follow the same pattern to organize you test cases.
Write independent test cases
Your test cases should not have dependency on other test cases, i.e you should be able to execute your test case individually with having dependency on other test cases.
Write small test cases
Always mention purpose of each test case clearly in test case.