An introductory dependent clause comes before the main clause and gives it context. In When the ferry docks, we’ll call you, the opening clause tells readers when the call will happen.
Find the clause and place the comma
Look for a subordinating conjunction such as when, because, or although. In the example, the ferry is the subject of the opening clause and docks is its verb. Put a comma after the entire clause, just before we’ll. The same time clause usually takes no comma at the end. We’ll call you when the ferry docks.
Opening clause or opening phrase?
After the ferry docks has a subject and a finite verb. After the ferry ride is a prepositional phrase with no verb of its own. Both can introduce a sentence, but the first is a dependent clause.
