[...] A subordinating conjunction signals the reader that the clause it's part of is dependent--common sub. conjunctions include before, after, while, unless, if, as, and because. The relevant rule is easy and well worth remembering: Use a comma after the subordinating conjunction's clause only if that clause comes before the independent clause that completes the thought; if the sub. conj.'s clause comes after the independent clause, there's no comma. Example: "If I were you, I'd put down the hatchet" vs. "I'd put down that hatchet if I were you."