What Are the Most Common Grammar Mistakes?

The most common grammar mistake in American college writing is using the wrong word – 13.7% of all errors found, according to Lunsford & Lunsford's 2008 national study of 877 first-year papers sampled from colleges across the US. The same research delivers a bigger surprise: students today make errors at almost exactly the same rate as students in 1917 – they just make different ones. Here are the most common grammar mistakes, ranked by real data, plus what they cost in the real world.

What Are the 20 Most Common Grammar Mistakes?

The best data comes from the only national error censuses ever conducted. In 2006, Andrea and Karen Lunsford had roughly 30 trained coders analyze 877 first-year papers, drawn from a stratified national sample of US two- and four-year colleges, against a 40-error rubric (published 2008). The top 15, with the share of all errors each one caused:

Rounding out the top 20: comma splices (2.9%), pronoun-antecedent agreement (2.7%), poorly integrated quotations (2.7%), unnecessary or missing hyphens (2.5%), and sentence fragments (2.4%). Comma problems are the biggest error family by far – five of the top 20 are comma errors, and together they account for roughly a quarter of everything students get wrong.

Modern tool data tells a similar story. Grammarly's 2022 ranking of student mistakes – drawn from its own editor, which reviews billions of words a month – puts spelling, run-ons, fragments, and the missing comma after an introductory phrase at the top. The academic census and the software logs converge on the same culprits.

Has Grammar Actually Gotten Worse?

No – and this is the most quotable finding in the entire literature. When Lunsford & Lunsford (2008) lined up every comparable study going back to 1917, the error rate barely moved:

The 2006 figure drops to 2.30 per 100 words once spelling is excluded to match the earlier studies – “almost exactly the same as it has been during the last century,” in the authors' words. Their conclusion: student errors “are not more prevalent – they are only different.” Meanwhile the average first-year paper grew from 162 words in 1917 to 1,038 in 2006 – students now write six times as much at the same accuracy.

What did change is which mistakes dominate. In the 1986 census (Connors & Lunsford, published 1988), spelling outnumbered every other error by roughly 300% and was tracked separately. By 2006, spelling had fallen to #5 – and wrong word had jumped from #4 to #1. The 2008 paper attributes part of that swap to spell-checkers: the software fixes definately but happily replaces a misspelled “frantic” with “fanatic,” converting a spelling error into a wrong-word error.

One more panic debunked in passing: the 2006 coders found almost no instances of texting shorthand like “gtg” or “imho” – or even smileys – in formal papers. Students switch registers just fine.

Why Are Capitalization Errors Rising?

Capitalization is the clearest new arrival on the list. It didn't appear in the 1986 top 20 at all – but by 2006 “unnecessary or missing capitalization” was the #8 error nationally, with 1,168 instances (5.2% of all errors). It's also an old problem returning: capitalization was the #2 most common error back in Johnson's 1917 study.

The 2008 paper blames distinctly digital habits: Word's autocapitalize firing after abbreviation periods and going uncorrected; brand names like eBay and iPod normalizing internal caps; and ad headlines – arguably the text students read most – modeling “seemingly random capitalization.” Students also capitalize whatever feels important: the study found papers capitalizing “Baseball” throughout, or “High School Diploma” mid-sentence.

Those two habits map directly onto questions we cover in depth: school subjects and majors stay lowercase unless they're proper nouns (do you capitalize majors?), and “high school” is only capitalized as part of an official name (is high school capitalized?). In titles, the confusion runs the other way – knowing which words are not capitalized in a title is what separates clean headlines from random ones.

The #19 error, hyphen misuse, has the same digital-era flavor – the study points to confusion between “sign-up sheet” and “sign up here,” and to two-part verbs written as “log-in.”

What Do Grammar Mistakes Cost Professionally?

Outside the classroom, the numbers get expensive. In CareerBuilder's 2018 survey of 1,023 US hiring managers and HR professionals, 77% named typos and bad grammar an instant resume deal-breaker – the #1 reason, more than double the runner-up (an unprofessional email address, at 35%). A 2013 wave of the survey had already put automatic dismissal for typos at 58%.

The promotion link is striking. Grammarly's 2013 study of 100 LinkedIn profiles found that professionals who never reached director level made 2.5× as many grammar errors as director-level peers – and those promoted six to nine times in their first decade made 45% fewer errors than those promoted one to four times. Correlation, not causation, but the pattern held across the sample.

Sales feel it too. UK entrepreneur Charles Duncombe told the BBC in 2011 that a single spelling mistake can cut online sales in half – fixing one typo on his tightsplease.co.uk site doubled revenue per visitor. That's one company's internal before-and-after, not a controlled study, but it remains the canonical citation. A small 2019 Website Planet experiment pointed the same way: landing pages with typos saw bounce rates 85% higher, and clean Google ads drew up to 70% more clicks than versions with errors (the authors themselves call it “a quick test”).

Which Mistakes Do Teachers Actually Mark?

Here's the uncomfortable part: teachers catch less than half of these errors. In the 1986 study, teachers marked only 43% of the errors that trained raters found in the same papers; by 2006 that had slipped to 38%. Which errors get flagged is revealing – the most-marked error of 1986 was its/it's confusion, marked 64% of the time even though it was the least frequent error in the top 20:

The takeaway: frequency and judgment are two different rankings. Its/it's barely registers by volume, but it's the error readers punish hardest – which is exactly why it's worth fixing first.

What Mistakes Do Non-Native Speakers Make?

For English learners, the biggest dataset is the Cambridge Learner Corpus – over 50 million words of Cambridge exam scripts, error-coded by an expert team (as described by Cambridge University Press in 2020). The 2003 technical paper on its coding system reports scripts from learners of 86 different first languages.

The headline finding: learner errors are predictable from the first language. Per Cambridge's own corpus examples (2020), French speakers misspell company as “compagny” (from French compagnie), Spanish speakers write “confortable,” and Japanese speakers drop articles – “visit Tate Modern museum.” The patterns are consistent enough that Cambridge publishes free “common mistakes” factsheets for eight first languages, from Arabic to Korean.

That makes the learner list genuinely different from the native-speaker list above: articles, prepositions, and L1-driven spellings dominate, while native speakers trip over wrong words, commas, and capitalization. Same language, two different error profiles.

Sources

1. “Mistakes Are a Fact of Life”: A National Comparative Study — College Composition and Communication (NCTE), 2008
2. Frequency of Formal Errors in Current College Writing — College Composition and Communication (NCTE), 1988
3. Frequency of Errors in Essays by College Freshmen and by Professional Writers — College Composition and Communication (NCTE), 1990
4. The Nation's Report Card: Writing 2011 — National Center for Education Statistics, 2012
5. National Profile Report, Graduating Class 2024 — ACT, 2024
6. Employers Share Their Most Outrageous Resume Mistakes and Instant Deal Breakers — CareerBuilder / The Harris Poll, 2018
7. The State of Business Communication — Grammarly & The Harris Poll, 2022
8. Report: How Grammar Influences Your Income — Forbes, 2013
9. Spelling Mistakes ‘Cost Millions’ in Lost Online Sales — BBC News, 2011
10. Your Typo Is Costing You Extra on Your Google Ads Spend — Website Planet, 2019
11. Top 10 Student Writing Mistakes: Finals Edition — Grammarly, 2022
12. Understanding Common Learner Errors and the Cambridge Learner Corpus — Cambridge University Press ELT, 2020
13. The Cambridge Learner Corpus: Error Coding and Analysis — Corpus Linguistics 2003 (UCREL, Lancaster University), 2003

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