Subordinate clauses modify or add information to independent clauses in complex sentences. There are three main types of subordinate clauses: adjective clauses modify nouns, adverb clauses modify verbs, and noun clauses are used as subjects, predicates, direct objects, or objects of prepositions. Some key characteristics include adjective clauses beginning with relative pronouns like who or which, adverb clauses beginning with subordinate conjunctions like when or because, and noun clauses introduced by words like that, what, or whoever.