What are affixes?

Affixes are indeed morphemes that are added to words to modify their meaning or create new words. They can be categorized into prefixes (which are added to the beginning of a word) and suffixes (which are added to the end of a word). For example, in the word "happy," the suffix "-ness" transforms the adjective into a noun, resulting in "happiness." This process of adding affixes is essential in constructing various forms of words and expanding vocabulary.

The other options do not accurately describe affixes. Grammatical structures refer to the rules and systems that govern the organization of words in sentences, which is distinct from the concept of morphemes. Synonyms and antonyms relate to words with similar or opposite meanings, rather than to the addition of morphemes. Similarly, idiomatic expressions are phrases that have a figurative meaning different from their literal meaning and do not involve the morphological changes that affixes create. Thus, understanding affixes as morphemes that modify words is crucial in exploring word formation and language mechanics.