Morphology is the study of the internal structure of words, focusing on questions such as what a word is, what its parts are, how new words are formed, and what patterns exist across languages. Lieber explains that words are not always indivisible blocks, since many consist of morphemes—the smallest units of meaning—like cats, which contains the root cat and the suffix -s indicating plural. The chapter distinguishes between simple words (e.g., book) and complex words (e.g., unhappiness), while also showing how morphology connects with other areas of language such as syntax (sentence structure) and phonology (the study of sounds). It addresses common misconceptions, such as the idea that every word has only one meaning or that only long words are morphologically complex, and emphasizes that studying morphology is essential to understanding language as an organized, creative, and systematic system, which is crucial for learning new languages, analyzing texts, and teaching grammar more effectively. 

This chapter explores what words are from both a linguistic and cognitive perspective, showing that they are not just sequences of sounds or letters but complex mental representations stored in the mental lexicon, our internal “dictionary” that includes information about pronunciation, meaning, grammatical category, and word relations. Unlike a physical dictionary, the mental lexicon is highly dynamic, organizing words by form, meaning, frequency, and morphological connections, with entries linked in networks such as happy, unhappy, and happiness. Lieber explains how new words enter the lexicon through processes like borrowing, derivation, and compounding, while dictionaries serve only as external references. By examining lexical entries containing phonological, syntactic, semantic, and morphological information, the chapter emphasizes how the mental lexicon underpins our ability to recognize, produce, and learn words quickly, making it a key concept in understanding both morphology and language cognition. 

Lieber introduces two fundamental processes of word formation—derivation and compounding—as the most common ways new lexemes are created. Derivation involves adding affixes to base words, often changing their grammatical category, such as happy → unhappy or teach → teacher, and is governed by rules that restrict which affixes can attach to which bases. Compounding, on the other hand, combines free-standing words to form new units like toothbrush or mother-in-law, whose meanings can be either predictable or idiomatic. The chapter highlights how both derivation and compounding are productive processes that expand vocabulary and reveal the flexibility of language, while also raising questions about how to define what counts as a “word.” These processes show that word formation is both systematic and creative, central to the evolution and richness of human languages. 

This chapter examines how different morphological processes vary in productivity, with some affixes like -ness being highly productive in forming new words, while others are restricted or no longer active. Lieber emphasizes that productivity is a gradient property influenced by factors such as semantic transparency, phonological compatibility, frequency, and the blocking effect of existing words. Alongside productivity, the chapter explores linguistic creativity, where speakers coin innovative forms like Googleable or unfriend that may not yet appear in dictionaries but are widely understood. Through both intuitive judgments and corpus studies, Lieber shows how productivity and creativity explain the adaptability of language, allowing it to respond to new communicative needs while still being governed by structural constraints. 

Moving beyond derivation and compounding, this chapter presents additional word-formation processes that illustrate the diversity of morphological strategies. Conversion, or zero-derivation, shifts a word’s category without affixes, as in email (noun) → to email (verb). Back-formation creates new words by removing perceived affixes, as with editor → edit. Blending merges parts of two words to create forms like smog or brunch, while acronym formation condenses phrases into single lexical items such as NASA or laser. Lieber highlights how these processes enrich vocabulary, challenge traditional definitions of words, and reveal the creativity of language users, ultimately showing that word formation extends far beyond the most familiar mechanisms. 

Here, Lieber shifts focus to inflection, the morphological process that modifies words to express grammatical information without creating new lexemes. Inflectional morphemes signal features like tense, number, case, gender, aspect, and comparison, as in cat → cats, walk → walked, or fast → faster. Unlike derivation, inflection produces systematic sets of related forms—inflectional paradigms—rather than new words, and languages vary in how they realize these systems, with fusional languages combining multiple grammatical meanings in one affix and agglutinative languages using separate affixes for each meaning. The chapter underscores the central role of inflection in sentence structure and agreement, highlighting how it ensures grammatical coherence and makes communication precise and efficient.