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The word conjunction comes from a Latin root meaning “join.” In grammar, a conjunction is a joining word.
A subordinating conjunction is a joining word that connects two clauses of unequal value. (A clause is a word group containing a subject and a verb.) The clause that begins with the subordinating conjunction is called a subordinate (or dependent) clause. It is less important than the other clause, which is called the main (or independent) clause.
For example, consider these two clauses:
Although they are grammatically equal, these two independent clauses don’t express equally important ideas. The second clause seems to be the main idea, and the first clause seems merely secondary.
We can join these clauses of unequal value into a single sentence by placing the subordinating conjunction as at the beginning of the less important idea:
The first clause is now a subordinate, or dependent, clause. It is no longer grammatically equal to the main clause because it cannot stand alone. For that reason, it must be joined to the main clause so that the two clauses form a single sentence. (Left by itself, a subordinate clause is a fragment, a common type of sentence error.)
Some of the most common subordinating conjunctions are shown below, grouped into categories according to the relationships they express. (In the examples, the subordinate clauses are in square brackets, and the conjunctions are in italics.)
Cause: as, because, since
Comparison: as, than
Condition: if, provided (that), unless
Concession: although, (even) though, whereas, while
Manner: as, as if, as though
Place: where, wherever
Purpose: in order that, so that
Time: after, as, before, once, since, till, until, when, while
Other: that (used to introduce a clause that functions as a noun)
Some subordinating conjunctions (especially those expressing time) can also act as prepositions. As prepositions, they introduce a phrase without a subject and verb:
As conjunctions, they introduce a clause containing a subject and verb:
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