Subordinating conjunctions (because, although, when, if, since, unless, while, until) connect a dependent clause to an independent clause, establishing a relationship of cause, contrast, time, or condition between them. The subordinating conjunction signals that its clause cannot stand alone — it needs the main clause to complete the thought. Moving the dependent clause to the front of the sentence (Although she was tired, she kept running) changes emphasis but not meaning, and typically requires a comma after the introductory clause.
Practice combining pairs of simple sentences using different subordinating conjunctions and notice how each one changes the logical relationship. Then identify subordinating conjunctions in published writing and ask what would be lost if the clauses were separated into two sentences.
You already know what a dependent clause is: a clause with a subject and verb that cannot stand alone as a sentence. A subordinating conjunction is what creates that dependency. Words like *because*, *although*, *when*, *if*, *since*, *unless*, *while*, and *until* attach to the front of a clause and transform it from independent to dependent. Before: "She was tired. She kept running." After: "Although she was tired, she kept running." The subordinating conjunction does two things simultaneously — it signals the logical relationship between the clauses and it prevents the clause from functioning on its own.
The choice of subordinating conjunction is not arbitrary; each one encodes a specific logical relationship. Causal conjunctions (*because*, *since*) assert that one thing caused or motivated another. Concessive conjunctions (*although*, *even though*, *while*) acknowledge something that might seem to contradict the main clause but doesn't override it. Conditional conjunctions (*if*, *unless*) introduce hypothetical or contingent scenarios. Temporal conjunctions (*when*, *while*, *until*, *after*, *before*) situate the dependent clause in time relative to the main clause. Using the wrong subordinating conjunction can silently reverse the meaning of a sentence: "Because she was exhausted, she ran faster" makes a very different claim than "Although she was exhausted, she ran faster."
Subordinating conjunctions are also the main tool for controlling emphasis in complex sentences. The main clause — the independent one — receives the primary emphasis; the subordinate clause is supporting material. If you want to emphasize that someone kept running, make the running the main clause: "Although she was tired, she kept running." If you want to emphasize the exhaustion, rearrange: "She kept running, although she was exhausted" — but now the main clause is still the running. The rule is: whatever you put in the dependent clause, you're treating as background; whatever you put in the main clause, you're foregrounding.
Comma placement follows naturally from position. When the dependent clause comes first (fronted), it's an introductory element, and you follow it with a comma: "If it rains, we'll cancel the picnic." When the dependent clause comes last, attached to the end of the main clause, no comma is needed: "We'll cancel the picnic if it rains." The logic is the same you applied to introductory elements more generally — front-loading subordinate material sets up the main clause, and a comma marks the handoff.