Plain-language explanation.
Syntax is the study of how words are combined to form sentences. Every language has grammar rules — not the prescriptive rules of school grammar, but the unconscious rules all speakers follow automatically. Even children who have never been explicitly taught grammar follow these rules perfectly.
Core concepts and standard treatment.
Syntactic categories (noun, verb, adjective, preposition) combine into phrases (NP, VP, AP, PP) and clauses governed by phrase structure rules or context-free grammars. Constituency tests (movement, substitution, coordination) identify phrase boundaries. Argument structure links verbs to their semantic roles (agent, patient, recipient) and syntactic positions. Transformations (movement operations) relate surface forms to underlying structures — wh-questions, passives, and relative clauses are canonical cases.
Deeper theory, debates and edge cases.
Parametric variation across languages: head directionality (head-initial vs. head-final languages), null subjects (pro-drop parameter: Italian allows subject omission, English does not), polysynthesis, and ergativity illustrate systematic variation within UG constraints. Dependency grammar (Tesniere) represents syntactic relations as binary dependencies between word pairs — computationally efficient and popular in computational linguistics. Semantic compositionality relates syntactic structure to semantic interpretation: meaning of a complex expression is a function of its parts and their combination.
How it is applied in practice.
NLP syntactic tools: constituency parsers (CYK algorithm, chart parsing) and dependency parsers (arc-eager, arc-standard transition systems; graph-based parsers using neural scoring) are deployed in information extraction, question answering, and machine translation. Grammar checking in word processors and writing assistants (Grammarly, LanguageTool) applies computational grammar to assist writers. Legal interpretation often turns on syntactic ambiguity — the scope of modification (Oxford comma debates) has implications for contract meaning and statutory interpretation.