The prefix "un" is used in the English language to indicate negation or the opposite of a word. For example, adding "un" to the word happy creates the word unhappy, meaning not happy. This prefix can be applied to various adjectives and verbs to convey a lack of something or the reversal of an action. In addition to adjectives, "un" can also modify verbs. For instance, the word do becomes undo when "un" is added, meaning to reverse the action of doing. This prefix is a common linguistic tool that helps expand vocabulary by creating antonyms or indicating negation.