Derivational affixes are prefixes or suffixes added to a base word to create a new word with a different meaning or grammatical category. For example, adding the prefix un- to the word happy forms the word unhappy, which changes the meaning to its opposite. Similarly, adding the suffix -ness to happy creates the noun happiness, transforming an adjective into a noun. These affixes play a crucial role in expanding vocabulary and allowing for more precise expression. By using derivational affixes, speakers and writers can modify words to convey different ideas, such as changing verbs to nouns or adjectives to adverbs.