Comma before 'so'

Whether a comma belongs before "so" depends on which job the word is doing. When "so" is a coordinating conjunction — one of the FANBOYS (for, and, nor, but, or, yet, so) — it links two independent clauses and means roughly "therefore." In that role it takes a comma: "It was raining, so we canceled the picnic." Both halves could stand alone as complete sentences.

When "so" means "so that," it introduces a purpose and acts as a subordinating conjunction — and no comma is used: "She left early so she could catch the train." The clause after "so" explains why the action was taken, not what resulted from it. A quick test: if you can substitute "so that" and the sentence still makes sense, omit the comma; if you can substitute "therefore," use one.

Compare: "He studied hard, so he passed the exam" (result — comma) versus "He studied hard so he would pass the exam" (purpose — no comma). The same comma logic applies to "which": a comma precedes "which" when it opens a nonessential clause adding extra information ("The car, which was red, sped away"), while "that" introduces essential information and takes no comma.

Comma rules with "so," "which," and "that" are tested directly on the ACT English section, which loves asking whether a comma belongs before a conjunction. Learning the independent-clause test — comma plus FANBOYS joins two complete sentences — resolves most of these questions quickly.