This is one of the most important shortcuts you can know for the ACT® English Test and the SAT® Writing Test, and it can save you a huge amount of time. You can expect to encounter several punctuation questions testing it in one form or another on any given exam.
Comma + and/but = Period = Semicolon
These three constructions are grammatically identical, so if more than one of them appear in answer choices, you can automatically eliminate all of those options.
Why? Because the test can never include more than one correct answer. If two answers are grammatically identical, then neither can be chosen over than the other, and both must therefore be wrong.
For example, consider the question below. The sentence is relatively simple, but it’s useful to illustrate a point.
Since canines were first domesticated nearly 40,000 years ago, they have played an important role in many human societies.
A. NO CHANGE
B. ago, and they
C. ago. They
D. ago; they
Because a period, a semicolon, and comma + and are all interchangeable, we can actually eliminate (B), (C) and (D) without even looking at the sentence. That leaves (A) as the default right answer.
On the ACT, the rule is also frequently tested in “Which of the following is NOT?” format. In most cases, the incorrect answer will include a comma alone. For example:
Canines were first domesticated nearly 40,000 years ago — they have played an important role in many human societies ever since.
Which of the following is NOT an acceptable alternative to the underlined portion?
A. ago; they have
B. ago. They have
C. ago, they have
D. ago, and they have
Note that it does not matter whether you know anything about dashes. You just need to be able to recognize that the semicolon in (A), the period in (B), and comma + and in (D) are equivalent. As a result, all of these answers can be immediately eliminated, leaving (C) as the only option.