Common advice when designing an API is to experiment with using the planned API before you implement it.

TDD is one way of doing this exploration, where the exploration is codified into actual code using the API and including assertions about the behavior you intend to implement.

sure! write a few tests pretending your thing is already implemented, to capture what you want it to do. at this point it's a step beyond writing no test and just typing `YourThing.Do()` in a text editor. does it make sense? is it awkward? should it even be `YourThing` or `SomeOtherThing`? what the "unit" is of what you're testing might change, or its API might. you're basically just trying to get a sketch of what it's like for the user.

now, at the end of this, you'll have a clearer idea of the external API boundary, probably a clearer vision of how it should work, _and_ code you can test against. you've potentially just saved yourself the labour of writing the thing, realizing it needs to be redesigned, and rewriting it.