A subordinating conjunction (also known as dependent word or subordinator) is placed at the beginning of a subordinate (or dependent) clause to show the relationship between the dependent clause and the main sentence.
A subordinating conjunction also turns the clause into something that depends on the rest of the sentence for its meaning.
Subordinating conjunctions are adverbs placed at the front of a clause to make it subordinate or dependent. The subordinate clause can come either before or after the main clause. If the the subordinate clause comes before the main clause, a comma is required.
Subordinators are usually a single word (after, as, because, once, than, since; etc) but they can also consist of a combination of words (as if, as long as, even if, in order that etc.) They can refer to: