A clause is a syntactic unit built around a verb (or verb group) that expresses a proposition. It is the fundamental unit of grammar in most linguistic traditions and the primary unit of analysis in Halliday's Systemic Functional Linguistics.
Main (independent) clauses can stand alone as complete sentences: The train arrived late.
Subordinate (dependent) clauses cannot stand alone and function within a main clause:
Finite clauses contain a verb marked for tense and subject agreement: She writes every day.
Non-finite clauses contain an infinitive, participle, or gerund with no tense marking:
Non-finite clauses are a marker of syntactic maturity in writing. They allow information to be packed more densely, contributing to lexical density.
Coordination: linking clauses of equal status with and, but, or, so. Produces compound sentences. Common at lower proficiency levels.
Subordination: making one clause grammatically dependent on another. Produces complex sentences. Increased subordination ratio is a standard measure of syntactic development in L2 writing.
Embedding: inserting a clause within a phrase (typically a noun phrase): The book [that I read] was fascinating. The embedded clause functions as a modifier, not a separate clause in the sentence structure.