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Lucas Fink

SAT Grammar: How to Spot Run-On Sentences

Even if you’re not a grammar nut, you’ve probably heard the term “run-on sentence” before, and you probably know that they’re bad. Most elementary school teachers get that across. But not everybody knows what they really are or how to avoid them. And that’s dangerous—SAT grammar expects you to know how to avoid run-ons.


Combining simple sentences

SAT writing multiple choice questions love to give you complex sentences to pick apart or simple sentences to combine (in improving paragraphs questions). Joining simple thoughts into clearly flowing, logical constructions is one of the most important factors of good writing, and it’s for that purpose that we have the rules of grammar that the SAT tests you on.

So if we have two simple sentences, like “My mother only eats goat meat and four-leaf clovers” and “I had a hard childhood,” there are a few different ways to stitch them together into one sentence. You might use a semi-colon, for instance. But let’s look at what not to do.


What are run-on sentences?

Sentences can’t just be thrown together into a single string and treated as one thought.

I had a hard childhood my mother only eats goat meat and four-leaf clovers

If you can put a period between two pieces of a sentence without making a fragment, then they can’t just be put together without some kind of grammatical glue.

That’s usually pretty clear—the big problem is knowing which glue to use.


Don’t combine whole sentences with a comma

A comma is not a period.

I can’t hear you, can you please use the megaphone?

I can’t hear you. Can you please use the megaphone?

If you can put “and,” “or,” “but,” or “so” after the comma, though, you’re alright.

I can’t hear you, so can you please use the megaphone?

Using a comma in place of a period is called a “comma splice,” and it’s one of the most common writing errors that high-schoolers make. And it’ll come up a few times in improving sentence or identifying sentence error questions on your SAT, so don’t forget it.


“And” alone is not enough

Although using a comma and “and” together to join sentences is correct , “and” alone doesn’t finish the job. Instead, it leaves you with the same problem that using a comma alone created: a run-on sentence. The same is true for “but,” “or,” and “so—all of them need commas to combine sentences.

Of course, all of those words also have other jobs. “And,” “but,” and “or” can just combine lists of nouns, verbs, or adjectives, and then they don’t need commas.

I bought glow-in-the-dark sunscreen and I went to the beach last night.

I bought glow-in-the-dark sunscreen and went to the beach last night.

Because there’s no “I” in the second half of that corrected sentence, the “and” is just combining “bought” and “went” into a short list, and that’s okay.


What to remember during your SAT: comma vs. period

If you see a comma or an “and” underlined in a writing multiple choice question (but not used together), ask yourself whether they can be replaced by a period. If they can, then there’s the problem.


P.S. Ready to get your highest SAT score? Start here.
About Lucas Fink

Lucas is the teacher behind Magoosh TOEFL. He’s been teaching TOEFL preparation and more general English since 2009, and the SAT since 2008. Between his time at Bard College and teaching abroad, he has studied Japanese, Czech, and Korean. None of them come in handy, nowadays.

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