What is a prefix?

A prefix is a specific type of affix that appears at the beginning of a word and alters its meaning. In this context, the correct answer indicates that a prefix is a morpheme that precedes a root and modifies its meaning. For instance, in the word "unhappy," the prefix "un-" modifies the root word "happy" to convey a different meaning, indicating the opposite emotion. This highlights how prefixes can create new words by adding particular nuances or altering the meanings of their root words, thus enhancing vocabulary and understanding in language.

The other options do not accurately describe prefixes: a word that cannot stand alone refers to a different concept in linguistics, often related to bound morphemes; a morpheme that comes after the root word describes a suffix, not a prefix; and a type of grammatical rule does not pertain specifically to prefixes. Understanding what prefixes are and how they function is essential for grasping the structure and meaning of words in language.