Follow the conventions of your behavior-driven development (BDD) framework. There is no mandatory grammar from a project management standpoint.
Scrum is Agnostic
Scrum doesn't prescribe a format for user stories. In fact, Scrum doesn't formally require the use of user stories or acceptance criteria at all. These types of artifacts are borrowed from methodologies like extreme programming, test-driven development, and the like.
Follow Your BDD Framework Conventions
How you write your tests will depend on your acceptance testing framework. For example, Cucumber uses the Gherkin language, which has
then keywords. These keywords guide the grammar and syntax of your stories, features, and scenarios. For example:
Given that I am using Cucumber,
When I write a Cucumber acceptance test in Gherkin
Then it will be written in the present- or future-perfect tense.