Dashes are a form of punctuation that is pretty much guaranteed to show up on both the ACT® English Test and the multiple-choice SAT® Writing Test. Because they tend to be used more frequently in British than in American English, they are typically the least familiar type of punctuation for many students. That said, they are relatively straightforward.
Dashes are tested in three ways. The first is extremely common, the second less common, and the third rare.
1) To set off a non-essential clause (2 Dashes = 2 Commas)
In this case, dashes are used exactly like commas to indicate non-essential information that can be removed without affecting the basic meaning of a sentence. If you have one dash, you need the other dash. It cannot be omitted or replaced by a comma or by any other punctuation mark. This is the most important rule regarding dashes that you need to know.
Incorrect: John Locke–whose writings strongly influenced The Declaration of Independence, was one of the most important thinkers of the eighteenth century.
Correct: John Locke–whose writings strongly influenced The Declaration of Independence–was one of the most important thinkers of the eighteenth century.
You can assume that almost every ACT, and most SATs, will contain at least one question testing dashes this way.
2) To introduce an explanation or a list (Dash = Colon)
In this case, a full, stand-alone sentence must come before the dash. The information that follows the dash does not have to be a full sentence (although it’s perfectly fine if it is).
Correct: John Locke was one of the most important thinkers of the eighteenth century–his writings strongly influenced The Declaration of Independence.
The information after the dash explains why Locke was one of the most important thinkers of the eighteenth century.
3) To create a dramatic pause
Finally, dashes can be used to create a break in a thought–they force the reader to stop for a fraction of a second before continuing on to whatever idea comes next. They are used to create a slight sense of drama or suspense.
Grammatically, this use is more or less interchangeable with #2: a full, standalone sentence must come before the dash, but either a sentence or a fragment can follow.
Correct: A number of John Locke’s ideas influenced The Declaration of Independence–particularly those concerning government, labor, and revolution.
To reiterate, this usage is not tested often, and you should simply be aware that it is acceptable.