This chapter shows the structure and use of sentences which contain more than one clause or lexical verb, and the difficulties such sentences can cause for learners of English. A simple sentence consists of one clause. Sentences which consist of more than one clause are called complex sentences. If one of the clauses does not begin with a conjunction, then it is the main clause, and the other are considered to be subordinate to the main clause. If a subordinate clause functions in place of a noun or noun phrase it is called a nominal clause. If a subordinate clause functions as an adjective or adjective phrase it is called a relative clause. If a subordinate clause functions as an adverbial it is called an adverbial clause. If a subordinate clause is used for making a comparison it is called a comparative clause. A non-finite clause can function as a relative clause which postmodifies a noun.