A Subordinating Conjunction (sometimes called a dependent word or subordinator) comes at the beginning of a Subordinate (or Dependent) Clause and establishes the relationship between the dependent clause and the rest of the sentence. It also turns the clause into something that depends on the rest of the sentence for its meaning.

• He took to the stage as though he had been preparing for this moment all his life.
  
  
  
• Because he loved acting, he refused to give up his dream of being in the movies.
  
  
  
• Unless we act now, all is lost.

Notice that some of the subordinating conjunctions in the table below — after, before, since — are also prepositions, but as subordinators they are being used to introduce a clause and to subordinate the following clause to the independent element in the sentence.