Here's why:
* Subordinate clauses cannot stand alone as complete sentences. They depend on a main clause to make sense.
* Main clauses contain a subject and a verb and can stand alone as a complete sentence.
For example:
Before we went out, we finished our homework.
* "Before we went out" is the subordinate clause, and it tells us the time frame for the main clause.
* "We finished our homework" is the main clause, and it expresses the main action.