Commas and coordination: using commas correctly with coordinating conjunctions, in lists, after introductory elements, and not between a subject and its verb, on Digital SAT boundaries questions.

What this skill is asking

Commas are the most frequently tested punctuation on the Digital SAT, and they follow a small set of rules. A Digital SAT boundaries question often turns on whether a comma belongs, and where. On the Digital SAT, the College Board (Standard English Conventions domain) tests the comma plus coordinating conjunction for joining two independent clauses, commas in a series, commas after introductory elements, and the rule that a comma must not split a subject from its verb. Knowing these four uses, and the cases where no comma belongs, answers most comma questions.

The four comma uses

Most correct commas on the SAT fall into one of these uses; most wrong commas fall outside them.

The mirror image matters just as much: a comma does not belong between a subject and its verb ("The team, won" is wrong), between a verb and its object, or before a conjunction that joins only two items ("eggs, and flour" is wrong when only two items are listed).

Two clauses versus two items

The most common comma error on the SAT is confusing a conjunction that joins two independent clauses (needs a comma before it) with one that joins two items or two verbs (needs no comma).

Introductions and series

After an introductory element, a comma sets it off from the main clause: "In the morning, the markets opened" and "Because the road flooded, traffic stopped." In a series of three or more, commas separate the items, and the SAT consistently uses the comma before the final "and" (the serial or Oxford comma): "red, white, and blue." A comma does not go before the first item or after the last. These rules are mechanical, which is why comma questions, like all boundaries questions, reward memorising the patterns from sentence boundaries and clauses and applying them quickly.

A reliable way to test a doubtful comma is to read the sentence aloud in your head and ask what the comma is doing: is it joining two complete sentences (then it needs a conjunction or should be a semicolon), separating list items, setting off an introduction, or enclosing a supplement? If the comma does none of those four jobs, it is almost certainly wrong. This single question, "which of the four jobs is this comma doing," resolves the large majority of comma choices on the section, because the SAT's incorrect commas are precisely those that do not fit any valid use, most often a comma wedged between a subject and its verb or floating before a conjunction that links only two verbs.