Boundaries
These questions test where one idea ends and another begins — how to join clauses correctly, and when to set information off with commas.
Two independent clauses (each could stand alone as a full sentence) can be joined a few ways: a period, a semicolon, or a comma plus a coordinating conjunction (and, but, or, so, yet, nor, for). A comma by itself is never enough — that’s a comma splice, one of the most commonly tested errors on the SAT.
A semicolon works well when both clauses are independent and closely related in idea — it’s a stylistic choice as much as a rule, so don’t reach for it just because two clauses happen to be grammatically joinable.
Commas around a clause depend on whether that clause is essential to identifying what it describes. If the sentence still makes sense with the clause removed — it was just extra detail — use commas on both sides. If removing it would change who or what the sentence is actually about, don’t use commas at all.