The complex sentence occupies a central position in theoretical English grammar, as it reflects the speaker’s ability to express logically and semantically interrelated ideas within a single syntactic structure. A complex sentence consists of one independent (principal) clause and one or more dependent (subordinate) clauses, which are grammatically and semantically linked to the main clause. This article examines the theoretical foundations of the complex sentence, its structural and semantic classification, and provides a detailed analysis of complex sentences containing subject, object, predicative, attributive, and appositive clauses. The study aims to systematize the main approaches to the classification of complex sentences and to demonstrate the functional diversity of subordinate clauses in modern English.