Word formation rules systematically build complex words from simpler elements through affixation, compounding, or internal change. Rules vary in productivity—some actively form new words (un- + adjective → opposite meaning), while others apply only to lexicalized forms. Productive rules follow regular patterns; unproductive rules show idiosyncratic semantics and restrictions.
From your study of morpheme types, you know the difference between free morphemes (words that stand alone: "run," "happy") and bound morphemes (affixes that must attach to a host: "-ness," "un-," "-ing"). Word formation rules describe how these elements combine systematically to produce new words. The central question in morphology is not just *what* combinations exist in the lexicon, but *why* some patterns apply broadly and others are frozen in only a handful of words. That question leads to the concept of productivity — the capacity of a rule to generate new words that speakers will recognize and accept.
A productive rule applies freely to new inputs. The prefix "un-" combines with adjectives to form negatives: "unhappy," "unclear," "unreliable." If you encounter a new adjective — say, "unpixelated" — you immediately understand it and accept it as well-formed, because the rule is active. Productivity is measurable: the more frequently a morphological pattern generates hapax legomena (words appearing only once in a large corpus), the more productive it is, because those one-off formations demonstrate speakers are applying the rule creatively. An unproductive rule, by contrast, applies only to lexicalized forms already stored in memory. The suffix "-th" forms nouns from adjectives: "length," "warmth," "depth." But you cannot productively apply it to new adjectives — "fastth" or "boldth" are not English words, even though the underlying pattern is transparent. The rule has fossilized.
The major word formation processes each have different scope. Affixation is the most systematic — prefixes and suffixes attach to bases and change either the meaning ("re-" → again) or the grammatical category ("-ness" converts adjective to noun, "-ize" converts noun or adjective to verb). Compounding combines two free morphemes into a single lexical unit: "blackbird," "sunlight," "deadline." Compounds differ from phrases: "blackbird" (a specific species) is not the same as "black bird" (any dark-colored bird), demonstrating that morphological combination creates new meanings that aren't simply the sum of parts. Conversion (also called zero-derivation) shifts a word's grammatical category without adding any affix: "to google," "to bottle," "to bookmark" — all nouns repurposed as verbs.
The most important practical skill is diagnosing *why* a rule is productive or restricted. Productive rules tend to be semantically transparent and compositional — the meaning of the output is fully predictable from the parts. Unproductive rules often have idiosyncratic semantics: "warmth" doesn't quite mean "the state of being warm" in the same regular sense that "happiness" means "the state of being happy." Lexical competition also blocks productivity: "furious" has no productive "*furiousness*" because "fury" already occupies that slot. These blocking effects reveal that the mental lexicon isn't just a rule applier — it caches existing forms, and an existing form can preempt a newly generated one. Mastering word formation means understanding both the generative rules and the constraints — lexical, semantic, and phonological — that limit their application.