To pretend is to act as if you're someone else. If you pretend to be your mother for a phone conference with your math teacher, try putting on her high heels to help you get into character.
When children or actors are pretending, it's fine, just playing. But if you pretend to someone who doesn't know you're just playing, you're likely to be condemned for it. No one likes it of you pretend to be a good member of the community just so people won't suspect you when you steal things from their houses. You might pretend you went to Harvard to impress someone, but if he finds out you didn't, he'll be very unimpressed.