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# Mark a form field as mandatory if a value entered in another field meets a condition

Form expressions allow you to build dependencies between fields based on defined calculations. For example, it is possible to make a text input component required if the variable `age` is defined in another component as greater than 18. You would add the expression `{{age>18}}` to the **Required** field of the text input component.

<img src="/files/pgN4oBY0nvVuEYhAwAyk" alt="" width="75%">