Commas After a Long Introductory Element

A sentence can start with a runway before it takes off: a phrase or clause that sets up time, place, or condition before the main subject and verb show up. 'After the long delay caused by the storm, the flight finally boarded.' Everything before the comma is scene-setting. The comma tells the reader: the setup is done, here comes the actual subject and verb. Without it, the words can run together and the reader has to backtrack to figure out where the real sentence begins.

To find where the comma goes, hunt for the main subject and verb first. Strip away the opening words and ask what is doing the action. In 'In the years following the fire the town rebuilt its main street,' the action is 'the town rebuilt.' Everything before 'the town' is the introduction, so the comma lands right after 'fire': 'In the years following the fire, the town rebuilt its main street.' Same test works whether the opener is a phrase or a full dependent clause with its own subject and verb, like 'Although the mill had employed nearly two hundred people for a century, it closed within a single season.' The clause has its own subject ('the mill') and verb ('had employed'), but it still can't stand as a complete thought alone, so it still gets closed off before the independent clause takes over.

Length and weight are what trigger the comma. A short opener like 'Today' or 'In 1990' doesn't confuse anyone about where the sentence starts, so the comma is optional there. This topic sits next to comma splices and interrupters in the same family of joint-marking: all three ask the same question, which words form the core sentence, and which ones are extra material needing a seam.