When a sentence opens with a dependent clause, ACT English wants a comma before the main clause starts. After the storm passed, we walked to the pier. Without that comma, the setup runs into the sentence. How clauses work is in independent and dependent clauses.
What counts as the opener
A dependent opener cannot stand alone. Before the last bell, when the lab closed, and if you finish early all need a comma when they come first. The same words at the end usually do not: We walked to the pier after the storm passed.
A short prepositional phrase is the same idea. In the second paragraph, the writer introduces the coach. The comma marks where the real sentence begins.
What this comma is not
It is not the comma inside a list. That job is covered in commas in a series. It is also not a comma between two full sentences. Two independent clauses still need a conjunction or a semicolon. The full set of comma jobs is in ACT comma rules, and the wider punctuation list is in ACT punctuation rules.
The bottom line
If the opening chunk cannot be its own sentence, put a comma where the main clause starts. If you move that chunk to the end, drop the comma.
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