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In Cucumber Behavior Driven Development, whatever you write must go into Given-When-Then steps.
Example:
Given that a library is added to simply And I am logged in
When I shall press get Or I press borrow Or I press reserve And I have internet connection And My account has no fines And My account has not expired
Then Book is on my bookshelf
Gherkin is the a defined syntax for the above test scenario. It is a set of special keywords to give structure and meaning to executable spec
More can be learned here https://cucumber.io/docs/gherkin/