Why does hitchstory mandate the use of given but not when and then?

Given-When-Then is a structured way of writing test cases or executable specs. It was invented by Dan North as part of behavior driven development.

Hoare logic

This pattern is a structure that essentially all test cases should be written, separating the preconditions from the actions from the observable outcome. It follows from Hoare logic - a means of reasoning rigorously about the correctness of a computer program.

Many BDD frameworks have explicit keywords for given, when and then. These keywords are intended to describe the structure of the story in a 'readable' way.

But what does given, when and then actually do?

Crucially, though, keywords don't actually do anything. In Cucumber, for example, there is no meaningful difference to putting "when" and "then" - they are essentially null operators.

I view them as something akin to test case writing "training wheels" - they are useful for beginner testers to keep the tester/specifier on track, ensuring that the test cases created are meaningful and actually test.

However, while training wheels are useful for beginners, they become cumbersome and get in the way once you no longer need them.

Terseness and clarity

Hitchstory recognizes that the pattern describes the structure of how should be written but, other than given, it neither requires nor encourages that the actual keywords be used.

Terseness is a key principle of hitchstory - the idea that stories should be as short and non-repetitive as possible, provided it doesn't inhibit readability.

As an example, in this case, "click" is the "when" step and "goes kaboom" is the then step. The story is still clear without them:

given:
  box: red
steps:
- click: red button
- goes kaboom

Still optional

Nonetheless, if you choose, you can create steps that start with when and then:

Given:
  I have: red box
Steps:
- When I click: the red button
- Then it goes kaboom