A validation rule's formula describes the error condition. When it returns true, the save is blocked and the user sees the configured message. This direction is easy to reverse. Write down one invalid record and one valid record before writing the formula.
Use AND when several conditions must hold together, OR when any invalid condition is enough, and explicit blank checks when a missing value is the defect. Transition functions such as ISCHANGED and PRIORVALUE let a rule apply to a particular change rather than reject every later edit. A useful message names what the person must fix. The exercises test both the forbidden state and valid neighboring states, including the exact numeric boundary.