The document discusses subordination in sentences. It defines subordination as a grammatical strategy to communicate that one idea in a sentence is more important than another. Subordinate clauses can take the form of full clauses or reduced clauses. Full subordinate clauses exhibit a subordinator and grammatical functions, while reduced clauses are missing constituents like the subordinator, subject, or finite verb. The document provides examples of different types of subordinate clauses and their functions within main clauses.