Coreferent subjects of complex clauses do not act in the same way. One of the coreferent subjectsis dropped from the complex clause. However, there are some kinds of complex clausesthat coreferent subjects are not deleted. There are some factors that influence this distinction:person, tense, transitivity-nontransitivity, kinds of complex clauses. This issue is not researcheddirectly in the grammar books and articles.This article investigates the usage of coreferent subjects in the complex clauses. This articleaims to analyze this issue in the frame of formal grammar and intents to explain datas in details.