Functional testing verifies that the software functions according to requirements, focusing on user interactions, APIs, databases, and security. Non-functional testing evaluates attributes like performance, usability, reliability, and scalability.

I prioritize test cases based on the criticality of features, risk factors, and business impact. I focus first on user-facing components and core functionalities critical to the application's success.

I first reproduce the defect to understand its conditions. Then, I investigate logs, review code, and use debugging tools to trace the problem's origin. Collaboration with developers is crucial for deeper insights.

I maintain flexibility by updating test cases and plans regularly, staying in close communication with the development and product teams. Agile methodologies and continuous integration help accommodate changes efficiently.

I use tools like Selenium, JUnit, TestNG, and Appium because they provide robust frameworks for web and mobile testing, support multiple languages, and integrate well with CI/CD pipelines.

Once, while testing a financial application, I found a calculation error in transaction processing. I immediately documented the steps to reproduce it, informed the development team, and prioritized it for an urgent fix while ensuring thorough testing to prevent similar issues.