Which concept relates to the way parts of words combine to form new words?

The concept that relates to the way parts of words combine to form new words is morphology. Morphology is the study of the structure of words and how they are formed from smaller units known as morphemes. Morphemes are the smallest meaningful components of a word, which can be roots, prefixes, or suffixes. For example, the word "unhappiness" consists of the prefix "un-", the root word "happy," and the suffix "-ness." Morphology examines how these morphemes interact and combine to create new words or convey different meanings.

Understanding morphology is essential for various fields, including linguistics, language education, and natural language processing, as it provides insight into word formation and the relationships between words in a language.