A subordinate clause is considered a dependent clause, so when we do a brief schematic diagram of a sentence with a subordinate clause, that diagram will look something like one of these variations:
DC, IC.
IC DC.
In each case, a subordinate clause cannot stand alone grammatically or logically. In both senses, the subordinate clause is a fragment unless complemented by an independent clause.
Note, too, how paragraphs work. A comma is required after an introductory dependent (or subordinate) clause at the beginning of a sentence, but not required before a subordinate clause at the end of a sentence.
The Big Six
Any list of subordinating conjunctions will have thirty to forty different words on it, but six will prove most useful to you as an academic / professional writer.
Because
causality--Because of y, x does...
Because the students love to write, the professor assigned longer essays.
While
simultanous time and logic relationships
While the flag represents freedom to many Americans, for others it symbolizes more ambivalent values.
When
time and logical conditions
When compared to the Great Depression, last week's events strike fear into the hearts of many.
Although
contrast / change of course
Although this election has gone on for nearly two years, many voters remain undecided.
Even though
contrast / change of course / synonym for "although"
Even though I allow students to rewrite essays, some students choose not to do so.
If
conditional (often includes "then" to introduce the independent clause
If Barack Obama wins the general election, the United States will have its first African American president.
Two to be careful about: As and Since
As
While you can find a dictionary definition of "as" to suggest causality, you should avoid using "as" to substitute for "because": "
As I was alarmed by the Wall Street events of the week, I wrote my congresswoman a plea for bipartisan action."
Replacing "as" with "because" in that introductory clause much improves that sentence. "As" works best for simultanoues relationships--over time or logical spaces:
As I contemplated my place in the universe last Monday morning, the stock market plunged nearly 500 points.
Since
You can also find a dictionary definition to justify using "since" as a causality link, but "since" works best to indicate time sequencing.
"Since I love the Beatles, I have all of their albums" is not as effective as "Because I love the Beatles, I own all of their albums."
But "Since the Beatles broke up, I have not been able to sleep" works quite well because "since" is very useful for time sequences. (Although, certainly, the sentence describes a life we would all consider a little sad!).