In Chinese language, the sentence is divided into the simple sentence and the complex sentence. We often hear people say the complex sentence is composed of several simple sentences, which is as a matter of fact incorrect saying. However, it is reasonable to regard the complex sentence as the grammar unit has a higher level than the simple sentence. If we express the complex sentence as the form S1, S2, S3… the letter S stands for the clause while the comma is the pause among each clause.

In this case, we can only say the whole part of S1, S2, S3… as sentence, but not several simple sentences. In other words, the clause is not sentence strictly. However, teachers in Mandarin learning courses would also tell you that we can’t regard the clause as phrase because the connection of clause and clause in the complex sentence is not the relationship of phrase and phrase. It is improper for us to understand the relations between clauses as any kind of structural relations in phrases level such as subject-predicate relation and modification-center structure.

In all, the clause is something has a higher level than the phrase but not the same as the sentence. Let’s call it the abstracted sentence, which is to say, the part remained after omitting the pause parts in the independent sentence. Sometimes, the clauses are connected by conjunctions. Let’s take a specific example that 因为昨天晚上下了雨，所以地上有水. Usually, 因为and所以are not regarded as one complement of the cause but only the means of connecting the two clauses 昨天晚上下了雨and地上有水.

According to the traditional analysis method of Chinese language grammar, conjunction is taken as a kind of grammar means but not the structural element. Another condition that the students who learn Mandarin need to know is that some clauses have fixed positions while some others do not. The former condition includes preposition and postposition like 请了假，才能离开学校.

In the sentence 请了假，才能离开学校, 请了假is the clause of the type of preposition while 才能离开学校is the clause of the postposition type. In the complex sentence contains only two clauses, 请了假 takes the place of S1 but not S2 while 才能离开学校 takes the place of S2 but not S1.