What do we call a morpheme that must be attached to other morphemes?

The term for a morpheme that must be attached to other morphemes is known as a bound morpheme. Bound morphemes are not able to stand alone as independent words; they require attachment to a free morpheme to convey meaning. Common examples of bound morphemes include prefixes, like "un-" in "unhappy," and suffixes, such as "-ed" in "walked." These morphemes provide grammatical function or modify the meanings of words but cannot exist independently.

In contrast, free morphemes can stand alone and function as complete words, like "cat" or "run." Combining forms are often used in compound words or in technical or scientific terms, but they are not exclusively defined as morphemes that require additional attachments. Final stable syllables, while relevant in certain linguistic contexts, do not specifically denote a morpheme's dependency on other morphemes for meaning. Therefore, the identification of bound morphemes is pivotal in understanding the structure and formation of complex words within a language.