Negate a root word's meaning of a word by setting the prefix un- before it. Calling something "uncool" lets everyone know the result was disappointing. It is in vogue to label a coworker's actions unprofessional when they do not meet the standards of acceptable conduct.
Anticipate events by placing pre- in front as a prefix. A pregame interview sets up the listener for the events of the game itself. A prepaid ticket requires no payment at the door. When you are pre-approved for a credit card, the bank makes the decision based on advance information it has about your credit score and payment history. You must still demonstrate you have the income to justify that trust.
Create uniqueness in the tone by setting up the word with the non- prefix. If a church is non-denominational, it has an independent organizational structure. Dieters can be attracted to the non-fat label on diary products. Organizations announce they are non-profit to distinguish themselves from businesses making money for their owners.
Reverse your course of action with the de- prefix. Something that is decapitated just lost a valuable part. If you want to de-emphasize something, just shy away from it. Detoxification helps drug addicts reverse the toxic damage they have done to their bodies.