It's also important to note that, like commas (but unlike parentheses), if you have a dash separating a concluding idea from the independent clause, you don't need a second dash before the period—see how I didn't add a dash immediately before the exclamation point in the above sentence?—. <--That's Wrong.
There are certain cases in which a dash is preferable to a comma. For example, if the interrupting phrase contains internal commas, a dash will separate it from the rest of the sentence more clearly than would commas.
That said, given that the rules overlap, how is one to know on a test like the SAT or ACT whether to choose the answer choice with commas, parentheses, or dashes?
Three main guidelines:
1) You are choosing among limited answer choices.
For most of the grammar sections, the goal isn't to identify the only correct way to write a sentence, rather the only error-free option among the answer choices. Sure, there may be instances where you could swap out dashes for commas or vice versa, but if the test chooses to use commas, commas it is.
2) There will NEVER be two correct answers.
That might sound obvious, but think about the implications of this basic test-taking tenet. If it looks like you could go with either a comma or a dash and both are included in different answer choices, there's a 100% chance that you are missing something—an error—in one of the answer choices. Which ties in with the third guideline...
3) Don't mix-and-match.
This is not a polka-dots-and-stripes so-wrong-it's-right situation.
If you choose to go with stripes (ehem, dashes), don't close the phrase with a parenthesis—it would look weird, right?). <--That's Wrong.
Choose a lane, and stick with it.
If, as suggested in guideline two, it appears that more than one answer choice could work and one answer uses a dash and the other uses a different punctuation mark, re-read the portion of the sentence that has not been underlined. If it includes a dash, comma, or parenthesis to close off the phrase, that indicates to you which punctuation mark you should be using within the underlined portion.
There's more where this came from.
Click here for my explanation of comma splices.
Click here for my explanation of a type of averages math problem.
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