What are "subordinating conjunctions"?

Subordinating conjunctions, also known as subordinators or dependent words, are a subset of conjunctions used specifically to join a subordinate clause to an independent clause. The subordinating conjunction appears at the beginning or middle of a sentence and denotes which clause depends on the main idea for context ("Conjunctions").
The following groupings list examples for each type of subordinator ("Subordinators").

Comparison and Contrast: although, though, even though, while, whereas

Cause and Effect: since, so that, because

Time: after, when, until, whenever, before

Possibility: if, as if, whether, unless

Place and Manner: wherever, where, how

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A cartoon representation of a subordinating conjunction in action.

The Rules

Subordinate clauses must be paired with an independent clause. Otherwise, they will be sentence fragments (Simmons 2013).
When attaching subordinate and independent clauses, consider these practices:

If the subordinating conjunction appears at the beginning of the sentence, use a comma to separate the clauses.
If the subordinating conjunction appears in the middle of the sentence, do not use a comma to separate the clauses unless the subordinate clause is nonessential.

Usage

Subordinating conjunctions can be used to join two related ideas into a single sentence (Simmons 2013).
Example:
     "Alex is hungry. He did not have time to eat breakfast."
     becomes
     "Alex is hungry because he did not have time to eat breakfast."

They can also be used to correct run-on sentences.
Example:
     "Bonnie finished with school, she went to the park."
     becomes
     "After Bonnie finished with school, she went to the park."

Links

Coordination and Subordination
Commas with Nonessential Elements

References

"Conjunctions." Conjunctions. Capital Community College Foundation, n.d. Web. 06 Nov. 2013. <http://grammar.ccc.commnet.edu/grammar/conjunctions.htm>.
Simmons, Robin L. "The Subordinate Clause." The Subordinate Clause. N.p., 2013. Web. 06 Nov. 2013. <http://www.chompchomp.com/terms/subordinateclause.htm>.
"Subordinators." Linking Words. Study and Learning Centre, RMIT, 2005. Web. 06 Nov. 2013. <https://www.dlsweb.rmit.edu.au/lsu../content/4_WritingSkills/writing_tuts/linking_LL/subordinators.html>.

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