Standard English Conventions is the most learnable part of the digital SAT, because the rule set is small and it recurs. Students who read each option aloud and pick the one that "sounds right" are doing the hardest possible version of an easy task.
1. Two complete sentences need real punctuation
If both halves could stand alone, a comma is not enough. That is a comma splice, and it is the single most tested error on the section.
Legal joins: a period, a semicolon, a colon, or a comma plus a coordinating conjunction (and, but, or, so, for, nor, yet).
The storm knocked out power for two days, the whole block cooked on camp stoves.
The storm knocked out power for two days; the whole block cooked on camp stoves.
2. However is not a conjunction
"However", "therefore", "moreover", "nevertheless" and "consequently" are adverbs. They cannot join two sentences with a comma, no matter how much they feel like they should.
The starter looks dead after a month, however, a single feeding revives it.
The starter looks dead after a month; however, a single feeding revives it.
3. Interrupters need a matched pair
A nonessential phrase dropped into the middle of a sentence has to be fenced on both sides with the same mark: two commas, two dashes, or two parentheses.
The bell tower, which once summoned farmers from three miles away now stands silent.
The bell tower, which once summoned farmers from three miles away, now stands silent.
Mixing marks — a dash opening and a comma closing — is always wrong, and it appears often precisely because it is hard to see.
4. A colon needs a complete sentence in front of it
Colons introduce lists, explanations and examples, but only after something that could end with a full stop.
The cave keeper watches for: humidity, temperature, and the pace of the rind.
The cave keeper watches three variables: humidity, temperature, and the pace of the rind.
The first fails because "The cave keeper watches for" is not a sentence.
5. Nothing separates a subject from its verb
No single comma between a subject and its verb, however long the subject gets. Long subjects are exactly where this gets tested.
The number of hours a glassblower spends at the furnace each week, is not what determines skill.
The number of hours a glassblower spends at the furnace each week is not what determines skill.
The one habit worth building
Do not read the four options and pick the best-sounding one. Look at what actually differs between them — usually one mark in one position — then ask which rule governs that position.
The options are constructed so that at least two of them sound fine. Sounding fine was never the test.