Complements, adjuncts and predicator are the three main elements of the clause structure. This paper primarily aims at presenting a general classification of clause structure in Persian. In this context, transitive and intransitive structures in Modern Persian are also analyzed. In this research, five canonical Persian constructions are identified based on valency and transitivity. The paper also discusses the ways in which complements and adjuncts are distinguished from each other. Since the study is oriented towards comparison with English, similarities and differences between the two languages are also discussed. In both languages the basic functions of the clause are the same. However, Persian contains five canonical structures whereas as English has just four of them. While in both languages the subject is the first element of the clause, the order of the elements is different in the two languages. In canonical structures in Persian, the predicator is always the final element of the clause, while in English it always fills the second position. In the context of valency, both languages have the same functions. Intransitive clauses are either monovalent or bivalent. Monotransitive clauses are either bivalent or trivalent. English intransitive clauses are always trivalent. Complements in the two languages are distinguished by factors such as licensing, obligator "ness," category and prepositional phrase. These factors are applicable in distinguishing complements from adjuncts.