The one test that applies to all of them
Before touching any specific rule, do this test on every punctuation question: check whether the words immediately before the punctuation mark form a complete sentence on their own, and separately check whether the words immediately after it also form a complete sentence on their own. A complete sentence has a subject and a verb and does not depend on another clause to make sense. This single check eliminates most wrong answers before you even think about which mark to use.
When a semicolon is correct
A semicolon is correct only when both the part before it and the part after it are complete sentences by themselves, and the two ideas are closely related. Think of a semicolon as a stand-in for a period: if you could put a period there and end up with two grammatically complete sentences, a semicolon is allowed there too. If either side is not a complete sentence, the semicolon is wrong, no exceptions.
Example that works: 'The lab results were inconclusive; the researchers repeated the experiment the following week.' Both halves are full sentences. Example that fails: 'Because the lab results were inconclusive; the researchers repeated the experiment.' The first half starts with 'because' and is not a complete sentence on its own, so the semicolon is wrong there even though the sentence sounds fine read aloud.
When a colon is correct
A colon requires a complete sentence before it. What comes after the colon does not need to be a complete sentence: it can be a single word, a list, a phrase, or a full sentence, as long as it explains, defines, or lists something related to what came before. The part after a colon cannot stand alone as its own reason for the colon; the part before it is what has to be complete.
Example that works: 'The trip required three things: a passport, a visa, and a plane ticket.' The part before the colon is a full sentence, the part after is a list. Example that fails: 'The trip required: a passport, a visa, and a plane ticket.' Here, 'the trip required' is not a complete sentence, so the colon is wrong.
When a dash is correct, and the trap to watch for
A single dash can be used almost anywhere a colon is correct, meaning a complete sentence before it introducing something after it. A pair of dashes can also be used the way a pair of commas or parentheses would be used, to set off extra information in the middle of a sentence, as long as removing everything between the two dashes still leaves a complete, correct sentence.
The trap most students fall into is mixing punctuation types in a pair, such as opening extra information with a dash and closing it with a comma, or opening with a comma and closing with a dash. On the SAT, if extra information in the middle of a sentence is set off on both sides, both marks need to match: two commas, two dashes, or two parentheses, never a mix. A second trap is treating a dash like it never needs a complete sentence before it. When a single dash functions like a colon, introducing something, the same complete-sentence-before rule applies as it does for a colon.
Worked example combining all three
Sentence: 'The committee reached a decision quickly ___ they had reviewed every proposal in advance.' Options: a semicolon, a colon, a comma, a period disguised as one option reading 'quickly, they'.
First, check both sides. 'The committee reached a decision quickly' is a complete sentence. 'They had reviewed every proposal in advance' is also a complete sentence. Both sides are complete and closely related in meaning, which is exactly the condition for a semicolon. A colon would technically also require a complete sentence before it, but a colon signals that what follows explains or introduces the first part directly, like a definition or list, not just a related but separate fact. Here, the second sentence gives a reason, but is still fully independent, which favors the semicolon over the colon. A plain comma joining two complete sentences without a conjunction is a comma splice, which is wrong. The answer is the semicolon.
How to drill this fast
Ignore how the sentence sounds when read aloud. Mechanically check whether each side is a complete sentence, then apply the rule for whichever mark is in the answer choices. This is a skill that gets fast with repetition because the check is the same every single time.
StudyPebble's punctuation practice sets isolate semicolons, colons, and dashes separately at first so you can drill the complete-sentence check on each one before mixing them together the way the real test does.