Asubordinate clause must be combined with a(n) to be a complete sentence. phrase subject modifier independent clause

Answer;
An independent clause
A subordinate clause must be combined with an independent clause to be a complete sentence. 

Explanation;
A subordinate clause is also known as dependent clause is a kind of clause that does not convey a complete thought and cannot stand alone; therefore, it needs the independent clause to convey its message. In other words, it is not a complete sentence and must be attached to an independent clause to become a complete sentence.