A subordinating conjunction is a word that introduces a subordinate clause which cannot form a sentence on its own. The subordinating conjunction connects the subordinate clause, also called a dependent clause, to an independent clause to form a sentence.
The following examples show subordinate clauses in bold. Each subordinate clause always begins with a subordinating conjunction. The rest of each of the sentences not in bold is the independent clause, also called the main clause.
As can be seen, the above subordinate clauses are introduced by subordinating conjunctions: although, because, before, since, unless and whenever. A subordinating conjunction can come in more than one word as shown in the last example: even though. A subordinate conjunction can come at the beginning of a sentence. But the subordinate clause must not be used independently as a complete clause or a sentence.
More examples of subordinating conjunctions: after, as, if, lest, now, once, provided, supposing, than, though, till, until, when, where, whereas, wherever, whether, while
Subordinating conjunctions of more than one word: as long as, as much as, as soon as, as though, even if, if only, in order that, just as, now that, provided that, rather than, so that,
An independent clause can be turned into a subordinate clause
(For more examples of subordinating conjunctions, see List 12 - Conjunctions.)