Morphology and Word Formation › Morphology and Word Formation

Words look like atomic units when you read them on a screen, but they are anything but. The word unbelievably packs four meaningful pieces into twelve letters: un- (not), believe (the core meaning), -able (capable of being X-ed), and -ly (turns an adjective into an adverb). A speaker of English need not have memorized "unbelievably" as a unit; they can assemble it from parts they already knew. This ability to build and decompose words is called morphology, and it is one of the main reasons language is so productive: with a modest memory of roots and affixes, speakers can produce and understand millions of word forms they have never seen before.

Throughout Part V we treated words as tokens to be counted and embedded, and we saw how subword tokenization emerged from the vocabulary problem: you cannot enumerate every word form, so you break unfamiliar ones into pieces. But those pieces were chosen by frequency, not by meaning. Morphology tells us what the right pieces often are, why certain words are missing from our vocabularies and embeddings, and how a model that knows unlock can reasonably guess something about unlockable. In this chapter we build the linguistic foundation for word structure. The next chapter covers computational morphological analysis and generation in depth, and later chapters in Part VII move from the structure of words to the structure of sentences.