subordinating conjunction 
noun
A conjunction that introduces a subordinating clause, e.g. although, because. Contrasted with coordinating conjunction.Questions beginning with a subordinating conjunction or a relative pronoun are complete sentences....
- The independent clause must be connected to the dependent clause in a subordinating conjunction.
- You could also think of the subordinating conjunction as a prepositional phrase, and you always need a comma after a prepositional phrase that starts a sentence.