What do the rules of a language primarily allow speakers to do?

Concept. A language is a rule-governed system. A finite set of rules — phonological, morphological and syntactic — specifies how a finite stock of words may be combined into acceptable utterances. Because these rules apply recursively, one structure can be embedded inside another structure of the same kind, so a finite rule system defines an unlimited set of well-formed sentences. Linguists call this property generativity, or productivity.

Knowing a language therefore means knowing its rules, not knowing a list of its sentences. Chomsky terms this internalised rule knowledge linguistic competence and distinguishes it from performance, the actual output a speaker produces on a given occasion.

Application. Applied here, what a language's rules primarily give a speaker is the power to produce and understand sentences that are new to them — utterances never heard before, yet accepted by other speakers as well formed. A child who has internalised English word order can say "The purple elephant ate my homework" without ever having met that sentence, and can equally judge "Elephant purple my homework ate the" as ill formed. Both abilities flow from the same rule system, so the rules primarily allow speakers to generate new and grammatically correct sentences.

Cross-check. Test that account against the other descriptions of what rules do:

• Producing output entirely free of mistakes: fluent adult speakers with a complete rule system still make slips and false starts, so error-free output tracks performance conditions rather than the rule system itself.

• Reproducing only previously heard sentences: this is the habit-formation account, and it cannot explain a child's novel forms such as "goed" or "mouses", which the child never hears from adults but builds by applying a rule.

• Storing sentences as unanalysed wholes: a memorised stock is finite and fixed by exposure, whereas ordinary speakers produce and understand an unbounded number of sentences and can judge an entirely unfamiliar string as well formed or ill formed.

So the primary function of a language's rules is generative: from a finite system they yield an unlimited number of new, grammatically correct sentences.