You use expressions in many operations in Microsoft Access, including creating calculated controls, query and filter criteria, default values, validation rules, and macro conditions. Often, the easiest way to create an expression is to find an example of an expression that is similar to what you want to create, and then modify it to do what you want.
What examples do you want to see?
Examples of expressions used in forms and reports
Examples of expressions used in queries and filters
Examples of default value expressions
Examples of field validation rule expressions
Examples of macro condition expressions
Read more about what expressions are