'Clause' can be defined as a group of words containing at least one subject and one verb. One clause can simply be a sentence or one or more clauses are combined to form a sentence. For example, I went to school. is a clause: I is a subject and went is a verb. The clause is a sentence as well. In a sentence such as I know that he went to school. there are two clauses: one is I know that he went to school and the other is that he went to school. The former and the latter have I/know and he/went as their subject/verb, respectively. The former clause (i.e.,I know that he went to school) is called a main clause; on the contrary, the latter (i.e., that he went to school) is called an embedded clause.