Grammar is the system of rules that allows people to combine words into meaningful sentences and to understand the sentences they hear or read. It includes rules of word order, agreement, tense, and hierarchical structure.

Every human language has a grammar, even if it has never been written down. Children acquire the grammar of their native language with remarkable speed and without formal instruction, extracting patterns from the speech they hear. Noam Chomsky argued that this achievement implies an innate language faculty that constrains the form of possible grammars. Other researchers emphasize the role of statistical learning and social interaction. The debate continues, yet the complexity of the knowledge children attain remains striking.

Grammar is distinct from the prescriptive rules taught in school. Descriptive grammar captures the patterns that speakers actually follow; prescriptive grammar states the patterns that authorities prefer. A sentence can be grammatically well-formed in the descriptive sense while violating a prescriptive rule.

Grammar is the invisible architecture that turns a string of words into a structured, interpretable message.