The prefix "un-" is used in the English language to indicate negation or the opposite of something. For example, when we add "un-" to the word happy, it becomes unhappy, meaning not happy. This simple prefix helps us express the opposite meaning of various words, making our communication clearer. In addition to emotions, "un-" can also modify actions and states. For instance, the word do becomes undo when we add "un-," signifying the reversal of an action. This prefix is a powerful tool in language, allowing us to create new words and convey different meanings effortlessly.