This paper argues that the explanation about the adverbial clause in school grammar has not systematic consistency caused by not considering the related other grammatical modules. As school grammar explained, If all the conjunctive ending which is the only way to make a conjunction in Korean is the adverbial ending which is a kind of embeded ending, every conjunction in Korean would be the subclass of embedding. It is not a proper nor a general recognition on the adverbial clause in Korean. Furthermore, such a view is not adopted in the classification of the extending of sentence. This paper also deals with the problems of the division of coordinate clause and subordinate clause in school grammar. In school grammar, even though coordinate clause and subordinate clause are distinguished, they are commonly included in the adverbial clause. It means that school grammar lacks of the enough grammatical proof to classify and establish the category of the adverbial clause. Though It is a difficult to separate coordinate clause from subordinate clause in Korean, it is not reasonable that these two clauses are treated as the same category.