Why it matters: Having distinct primary and secondary actions means users won’t be confused while interacting with your website and will be less prone to make mistakes. For example, 'Submit' and 'Cancel' buttons must always be clearly distinct.
How to test it: During usability testing, pay attention to common errors that result from poorly distinct primary and secondary actions. When you review your design, make sure that colour, size, positioning and other elements differentiate these actions.
Just look at this example: