The Grammar Mistakes That Actually Cost You Marks

Most grammar advice tries to cover everything at once, which means it ends up covering nothing well. Here's a shorter, more useful list: the handful of mistakes that actually change how a reader judges your writing, ranked by how often they show up and how much damage they do.

Its vs It's

This one is almost embarrassingly common given how simple the rule is: "it's" is always a contraction of "it is" or "it has," never a possessive. "The company changed its policy" is correct because you're showing possession. "It's raining" is correct because you mean "it is raining." If you can't substitute "it is" into the sentence and have it still make sense, you need "its," not "it's." Grammarly's explanation of its versus it's covers this specific confusion in more depth, since it's consistently one of the most common errors in student and professional writing alike.

Comma Splices

Joining two complete sentences with just a comma, "the results were promising, the team decided to proceed," is a comma splice, and it's one of the fastest ways to signal an unpolished draft. Fix it with a period, a semicolon, or a coordinating conjunction: "the results were promising, so the team decided to proceed."

Dangling Modifiers

"Walking to the store, the rain started falling" technically says the rain was walking to the store, because the modifying phrase attaches to whatever noun comes right after the comma. The fix: "Walking to the store, I got caught in the rain." This mistake is common because it reads fine at normal speed; your brain fills in the intended meaning automatically, which is exactly why it's worth reading your own drafts more slowly than feels natural.

Mixing Up Homophones Under Deadline Pressure

"Their," "there," and "they're" get confused not because writers don't know the difference, but because typing speed outpaces the check. Same with "affect" (usually a verb, to influence) and "effect" (usually a noun, a result). These aren't knowledge gaps, they're proofreading gaps, and they're worth a dedicated final pass specifically hunting for homophones rather than trusting a general reread to catch them.

Why This Short List Matters More Than a Long One

A twenty-item grammar checklist is too long to actually use before submitting something. A five-item list you've genuinely memorized gets applied every time. Pick the mistakes you personally make most often, keep that list short, and check specifically for those before you submit, rather than trying to hold every grammar rule in your head at once. The Purdue OWL's general writing resources and UNC's writing tips are both worth bookmarking for the errors that don't make this short list but still show up occasionally in your own writing.

Grammar isn't about knowing every rule. It's about reliably catching the five mistakes you're personally prone to, every single time. If your writing regularly involves quoted or paraphrased material, keep an eye on how quotes interact with punctuation too, since misplaced commas around a quotation are their own common source of small errors, and UNC's guide to paraphrasing touches on this alongside its main subject. A quick check against Cambridge Dictionary for word-level usage rounds out the short list of habits worth building before you rely on any grammar checker to catch the rest.