What distinguishes a bound morpheme from a free morpheme?

A bound morpheme is defined by its inability to stand alone as a separate word; instead, it must be attached to a free morpheme to convey meaning. This attachment often alters the meaning of the word to which it is added. For example, in the word "unhappiness," the bound morpheme "un-" modifies the free morpheme "happy" to suggest a negation, while the suffix "-ness" transforms the adjective into a noun. This capacity to change or enhance the meaning of a base word is a critical feature of bound morphemes.

In contrast, free morphemes can exist independently as complete words, which is a key differentiator. Additionally, while bound morphemes do carry meaning, they do not have meaning on their own, and they can appear in various positions within a word, not strictly at the end. Thus, the option that emphasizes the meaning-altering function of bound morphemes is the most accurate statement regarding their role in language.