Common grammar mistakes

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Most grammar advice online mixes two very different things: real errors that make your writing harder to read, and old schoolroom superstitions that never reflected how good English works. This page separates them. The first section covers ten mistakes worth fixing. The second covers four "rules" you can retire with a clear conscience, along with the evidence.

Ten grammar mistakes worth fixing#

1. Comma splices#

A comma splice glues two complete sentences together with nothing but a comma. The Harvard Law School Writing Center calls it the most common kind of run-on and offers three repairs: add a coordinating conjunction after the comma, swap the comma for a semicolon, or end the sentence.

2. Subject–verb agreement across a long gap#

Nobody writes the dogs barks. The trouble starts when a phrase separates the subject from its verb and the verb latches onto the nearest noun. Harvard's guide advises checking that the verb agrees with its actual subject, not the words in between.

3. Apostrophes in plurals#

The "greengrocer's apostrophe" (apple's 50p) is the single most visible punctuation error. Purdue OWL is blunt: don't use apostrophes for noun plurals. It also says decades are written the 1960s, with no apostrophe before the s, and that capital letters and numbers don't need one to form a plural either.

4. It's vs its#

Covered in depth on our commonly confused words page, but it belongs on any grammar list. It's is only ever it is or it has; the possessive its takes no apostrophe (Cambridge).

5. "Between you and I"#

After a preposition (between, for, with, to), pronouns take the object form: me, him, her, us, them. So it is between you and me and a gift for my wife and me. The wrong version is a textbook case of hypercorrection: having been corrected for saying you and me went as children, people come to feel that you and I is always more proper. Rutgers English professor Jack Lynch described exactly this process in a Voice of America interview.

6. Who vs whom#

The traditional rule, as set out in the American Heritage usage note, is that who is a subject and whom is an object. The subtle part: when the pronoun is the subject of its own clause, it stays who even after a verb or preposition, as in I know nothing about who supports the governor. Cambridge notes that whom belongs mainly to formal styles and writing, and its own examples show who standing in for it in ordinary use (Anne Bolton, who he married).

7. Lay vs lie#

Lay takes an object (you lay something down); lie does not (you lie down). The past tense of lie is lay, which is why this pair has been confused, per the American Heritage Dictionary, since the 1300s. The dictionary's full set of correct forms:

See also Cambridge's lay or lie.

8. Fewer vs less#

Fewer for countable things, less for mass amounts. The American Heritage usage note also records that the supermarket sign 10 items or less got a slightly warmer reception from its Usage Panel than other violations, though still not majority approval, and that less than $400 and less than three weeks are standard.

9. "Could of" for "could have"#

In speech, could have shrinks to could've, and the unstressed 've sounds just like of. That's a normal feature of English weak forms, but in writing it produces could of, should of, would of, which are always errors.

10. Dangling modifiers#

A dangling modifier describes someone who never actually appears in the sentence. Purdue OWL recommends making the real doer the subject of the main clause, or rewriting the phrase as a full clause.

Four "rules" you can safely ignore#

Never end a sentence with a preposition#

This one has a specific author. The New Republic traces it to a tossed-off criticism of Ben Jonson by the poet John Dryden in 1672, later cemented by 18th-century grammarians who admired Latin, which doesn't strand prepositions. Linguist Geoffrey Pullum's notes on stranded prepositions point out that Dryden gave no reason at all. What are you looking at? is perfect English; At what are you looking? is a costume.

Never split an infinitive#

To boldly go was fine all along. Wikipedia's summary of the history notes that prescriptivists campaigned against split infinitives from the 19th century, but recent usage guides have largely dropped the objection. Sometimes splitting is the only way to put the adverb where it belongs: to more than double has no graceful unsplit alternative.

"Data" must be plural#

Both uses are standard. The American Heritage usage note says the plural survives mainly in scientific writing and that singular mass-noun data is standard for general audiences. It concludes it is "safe to say" singular data has become standard usage.

"Hopefully" can't start a sentence#

Hopefully, the bill will pass uses hopefully as a sentence adverb, just like frankly and mercifully. The American Heritage usage note observes that critics singled it out in the 1960s for no clear reason and that resistance has waned. If a reader you care about objects, I hope is always available.

A proofreading checklist#

• Search for every comma between two clauses and ask: could each side stand alone? If so, you have a splice.
• Find the verb, then find its real subject. Ignore the prepositional phrase in between.
• Search for 's and confirm each one is a possessive or a contraction, never a plural.
• Search for and I and try the drop-the-other-person test.
• Search for of after could, should, would, might, must.
• For each opening -ing phrase, check that the subject right after the comma is the one doing it.

Grammar errors cluster around the same handful of word pairs, so pair this page with commonly confused words and misused idioms and phrases. If your argument is the problem rather than your sentences, see logical fallacies.

Frequently asked questions

Can you end a sentence with a preposition?

Yes. The supposed rule traces back to a remark by the poet John Dryden in 1672, and linguists such as Geoffrey Pullum have shown that stranded prepositions are normal English used by great writers for centuries. Rewrite only if the sentence reads awkwardly.

Is it 'between you and me' or 'between you and I'?

Between you and me. Between is a preposition, and the pronoun after a preposition takes the object form (me, him, her, us, them). Between you and I is a classic hypercorrection.

What is a comma splice?

Two complete sentences joined only by a comma: The court intended to broaden standing, it did not clarify the limits. Fix it with a conjunction (, but), a semicolon, or a full stop.

Is 'data' singular or plural?

Both are standard. The American Heritage Dictionary notes that plural data remains common in science, while singular mass-noun data (like information) is standard in general writing; in its 2005 survey 66 percent of its Usage Panel accepted Once the data is in, we can begin to analyze it.

Is it wrong to split an infinitive?

No. Prescriptivists objected from the 19th century onward, but recent usage guides have largely dropped the objection. To boldly go is fine; move the adverb only if the result reads better.

Sources

Every factual claim on this page links to one of these. Checked October 2026. See how we source.

1. Harvard Law School Writing Center: Common language errors hls.harvard.edu
2. Purdue OWL: Apostrophe introduction owl.purdue.edu
3. Purdue OWL: Dangling modifiers and how to correct them owl.purdue.edu
4. Cambridge Dictionary: It's or its? dictionary.cambridge.org
5. Wikipedia: Hypercorrection en.wikipedia.org
6. VOA Learning English: Wordmaster on hypercorrection learningenglish.voanews.com
7. American Heritage Dictionary: usage note at 'who' ahdictionary.com
8. Cambridge Dictionary: Who, whom dictionary.cambridge.org
9. American Heritage Dictionary: usage note at 'lay' ahdictionary.com
10. Cambridge Dictionary: Lay or lie? dictionary.cambridge.org
11. American Heritage Dictionary: usage note at 'less' ahdictionary.com
12. Wikipedia: Stress and vowel reduction in English (weak forms) en.wikipedia.org
13. Geoffrey K. Pullum: Stranded prepositions lel.ed.ac.uk
14. The New Republic: The dangling preposition myth newrepublic.com
15. Wikipedia: Split infinitive en.wikipedia.org
16. American Heritage Dictionary: usage note at 'data' ahdictionary.com
17. American Heritage Dictionary: usage note at 'hopefully' ahdictionary.com