The rhythm of TDD is to first write a failing test. That starts driving the design of your production code. To do that you need to invoke a function/method with arguments that responds with an expected answer.

At that point you've started naming things, designing the interface of the unit being tested, and you've provided at least one example.

Let's say you need a method like isEven(int number): Boolean. I'd start with asserting 2 is even in my first test case.

To pass that, I can jump to number % 2 == 0. Or, I can just return true. Either way gets me to a passing test, but I prefer the latter because it enables me to write another failing test.

Now I am forced to write a test for odd input, so I assert 3 is not even. This test fails, because it currently just returns true. Now I must implement a solution that handles even and odd inputs correctly; I know modulus is the answer, so I use it now. Now both tests pass.

Then I think about other interesting cases: 0, negative ints, integer max/min, etc. I write tests for each of them, the modulus operator holds up. Great. Any refactoring to do? Nope. It's a one-liner.

The whole process for this function would only add a few minutes of development, since the implementation is trivial. The test runtime should take milliseconds or less, and now there is documentation for the next developer that comes along. They can see what I considered (and what I didn't), and how to use it.

Tests should make changing your system easier and safer, if they don't it is typically a sign things are being tested at the wrong level. That's outside the scope of this lemmy interaction.