A common prefix is a group of letters that are placed at the beginning of words to create new words. The three common prefixes PRE, MIS, and NON all have three letters each and are used to modify the meaning of words. PRE means 'BEFORE' and is used to indicate something that happened BEFORE something else. MIS means 'WRONG' and is used to indicate something that is incorrect or WRONG. NON means 'not' and is used to indicate something that is not true or does not exist.