When a dependent clause comes at the beginning of a sentence (before the independent clause), it must be followed by a comma: 'When she arrived, we started the meeting' and 'Although the weather was cold, the children played outside.' This comma separates the introductory dependent clause from the main clause. However, when the dependent clause comes after the main clause, no comma is needed: 'We started the meeting when she arrived.' The purpose of the comma is to make the sentence structure clear to the reader.
You already know from studying dependent clauses that they cannot stand alone — they need an independent clause to complete their meaning. When a dependent clause opens a sentence, it creates a particular reading challenge: the reader encounters a clause that feels incomplete before the main point has arrived. The comma signals the boundary, telling the reader: "The introductory part is done; here comes the main clause you were waiting for." Without the comma, readers sometimes merge the two clauses in ways that create momentary confusion, even if the meaning eventually becomes clear.
The position of the dependent clause determines everything. Compare: "When the storm ended, we went outside" versus "We went outside when the storm ended." The meaning is identical, but the punctuation differs. In the first sentence, the dependent clause is introductory — it comes before the independent clause — so a comma is required. In the second, the dependent clause follows the independent clause, and no comma is needed. The rule is really about order: introductory dependent clause + comma + main clause, but main clause + dependent clause (no comma). A useful test: if you can flip the sentence so the independent clause leads and the meaning holds, the version with the dependent clause first needs the comma.
Common subordinating conjunctions — *although*, *because*, *when*, *if*, *since*, *while*, *even though* — are the signal words that typically introduce dependent clauses. When you spot one of these at the very start of a sentence, you're almost certainly looking at an introductory dependent clause that will need a comma. The comma comes at the end of the entire dependent clause, not right after the conjunction: "Although it was raining hard, the game continued" — the comma follows *raining hard*, not *although*. Many writers misplace the comma by inserting it too early, immediately after the subordinating conjunction.
The deeper logic behind this rule connects to how punctuation manages processing load for readers. When information arrives in an unexpected order (dependent before independent), readers need a clear signal to parse the structure correctly. The comma does that structural work. This is why the rule applies even to short introductory clauses where the "ambiguity" might seem minimal: "If you go, let me know." The comma is a courtesy to the reader, marking the transition from setup to main point. As your writing becomes more complex, this comma will become automatic — a reflexive signal you insert whenever an introductory dependent clause precedes the main idea.