Grammar and style

20 Common English Grammar Mistakes (and a 5-Second Test to Fix Each One)

Its or it's? Fewer or less? Who or whom? These are the mix-ups that slip into emails, posts, and even published articles. Here's how to spot each one fast, without memorizing a grammar textbook.

How Many Words October 23, 2026 12 min read

The short answer

The most common English grammar mistakes are mix-ups between words that sound alike, like its/it's, your/you're, and their/there/they're. Next come word-choice errors like fewer/less and affect/effect, and punctuation slips like comma splices and apostrophes in plurals.

Most can be fixed with a quick test. For example, if you can replace the word with "it is," write it's.

Most grammar mistakes aren't caused by not knowing the rule. They happen because you're typing fast, autocorrect jumps in, or two words sound exactly the same in your head.

That's why a short, reliable test for each one helps more than a long explanation. Below are 20 of the most common confusions in English, each with a wrong and right example, a quick test, and a reality check on how much it actually matters.

Context

Why these mistakes slip through more than ever

Phones and autocorrect. Predictive text happily suggests "your" when you meant "you're." On a small screen, you're less likely to notice.
Speed. Chat apps and quick replies reward fast typing, not careful proofreading.
Grammar tools and AI aren't perfect. They catch a lot, but they can miss errors that depend on meaning, and sometimes "fix" sentences that were already correct.
Global English. More people than ever write in English as a second or third language, in international teams and online communities. Some of these rules work differently in other languages.

Reality: Everyone makes these mistakes, including professional writers. The goal isn't perfection. It's catching them before you hit send.

Part 1 · Mistakes 1–7

Sound-alike words

They sound the same, so your fingers pick the wrong spelling. These are the most common mistakes in writing online.

1Its vs. it's

It's is short for "it is" or "it has." Its shows that something belongs to "it."

Wrong	Right
The app lost it's data.	The app lost its data.
Its been a long week.	It's been a long week.

Quick test: Replace it with "it is" or "it has." If the sentence still works, use it's.

Reality: Possessive pronouns like its, his, and hers never take an apostrophe.

2Your vs. you're

You're means "you are." Your shows that something belongs to you.

Wrong	Right
Your going to love this.	You're going to love this.
Thanks for you're help.	Thanks for your help.

Quick test: Say "you are" in its place. If it works, write you're.

Reality: This one is almost always a typing slip, not a knowledge gap. Proofreading catches it.

3Their vs. there vs. they're

Their shows ownership. There is a place or introduces something ("there is"). They're means "they are."

Wrong	Right
Their are three options.	There are three options.
The team shared there notes.	The team shared their notes.
There launching next week.	They're launching next week.

Quick test: There contains "here" (a place). Their contains "heir" (someone who owns). They're expands to "they are."

Reality: Grammar tools usually catch these, but not always. A quick read before sending is still the best check.

4To vs. too vs. two

Two is the number. Too means "also" or "excessively." To is everything else: direction, or before a verb.

Wrong	Right
This file is to large.	This file is too large.
I want to come two.	I want to come too.

Quick test: If you mean "also" or "more than enough," you need the extra "o": too.

Reality: "To" is the default. Only switch when you mean the number or "also."

5Then vs. than

Than compares things. Then is about time or sequence.

Wrong	Right
This draft is better then the last one.	This draft is better than the last one.
Write first, than edit.	Write first, then edit.

Quick test: Comparing? Use than. Describing what happens next? Use then.

Reality: "Better then" can change your meaning entirely. It's worth getting right.

6Lose vs. loose

Lose (one "o") means to misplace or fail to win. Loose (two "o"s) means not tight.

Wrong	Right
Don't loose your work. Save often.	Don't lose your work. Save often.
The cable is lose.	The cable is loose.

Quick test: Lose lost an "o."

Reality: Spell checkers won't flag this one, because both are real words.

7Affect vs. effect

In most everyday writing, affect is a verb (to influence), and effect is a noun (a result).

Wrong	Right
Screen time can effect your focus.	Screen time can affect your focus.
The change had a big affect.	The change had a big effect.

Quick test: Affect is an Action. Effect is the End result.

Reality: There are rare exceptions ("to effect change" means to bring it about), but the test covers almost every case.

Part 2 · Mistakes 8–12

Word choice

Both words are real, but only one says what you mean.

8Fewer vs. less

Use fewer for things you can count. Use less for amounts you can't count.

Wrong	Right
Use less words.	Use fewer words.
It takes fewer time.	It takes less time.

Quick test: Can you put a number in front of it ("three words")? Use fewer.

Reality: Signs like "10 items or less" are common, and nobody's confused by them. In formal writing, stick with fewer for countable things.

9Who vs. whom

Who does the action. Whom receives it, or follows a preposition like "to" or "with."

Wrong	Right
Whom wrote this?	Who wrote this?
To who should I send it?	To whom should I send it?

Quick test: Answer the question with he or him. "He wrote it" means who. "Send it to him" means whom. Both him and whom end in "m."

Reality: In casual writing, "Who should I send it to?" is perfectly normal. Save whom for formal writing.

10E.g. vs. i.e.

E.g. means "for example." I.e. means "that is" or "in other words."

Wrong	Right
Use a newsletter platform, i.e., Substack or Ghost.	Use a newsletter platform, e.g., Substack or Ghost.
Send it by the deadline, e.g., Friday at noon.	Send it by the deadline, i.e., Friday at noon.

Quick test: E.g. = "example given." I.e. = "in essence."

Reality: If you're unsure, write "for example" or "that is" in full. It's clearer for everyone, especially non-native readers.

11Complement vs. compliment

A complement completes or goes well with something. A compliment is praise.

Wrong	Right
The chart compliments the article.	The chart complements the article.
Thanks for the complement!	Thanks for the compliment!

Quick test: A complement completes. A compliment is something I like to hear.

Reality: Spell checkers can't help here. Both words are spelled correctly.

12Lie vs. lay

You lie down yourself. You lay something else down. The tricky part: the past tense of lie is lay.

Wrong	Right
I need to lay down for a bit.	I need to lie down for a bit.
Lie your phone on the desk.	Lay your phone on the desk.

Quick test: If there's an object ("lay the phone"), use lay. If not, use lie.

Reality: Even native speakers mix this up in speech. In writing, it's worth the extra second.

Part 3 · Mistakes 13–17

Grammar

Especially common for people writing in English as a second language.

13I vs. me

I does the action. Me receives it. People often get this wrong when another person is in the sentence.

Wrong	Right
Aya and me wrote the guide.	Aya and I wrote the guide.
Send it to Aya and I.	Send it to Aya and me.

Quick test: Remove the other person. "Me wrote the guide" sounds wrong, so use I. "Send it to I" sounds wrong, so use me.

Reality: "And I" isn't automatically more correct. It's only right when I is doing the action.

14A vs. an

The choice depends on the first sound of the next word, not the first letter. Use an before a vowel sound and a before a consonant sound.

Wrong	Right
a hour, an user	an hour, a user
a SEO tool	an SEO tool ("ess-ee-oh")

Quick test: Say the next word out loud. Hear a vowel sound? Use an.

Reality: Acronyms trip people up most. It's "an FAQ," "a URL," and "an API."

15Uncountable nouns

Some English nouns can't be counted directly, even if they can in other languages: advice, information, feedback, equipment, research, software.

Wrong	Right
Can you give me some feedbacks?	Can you give me some feedback?
I have an advice for you.	I have some advice for you. / a piece of advice

Quick test: If it feels like a substance or a general idea rather than a separate item, try "some" or "a piece of."

Reality: "Feedbacks" and "informations" are some of the most common mistakes in international teams. Easy to fix once you know.

16Since vs. for

Since points to when something started. For gives how long it's lasted.

Wrong	Right
I've written daily since three months.	I've written daily for three months.
We've worked here for 2024.	We've worked here since 2024.

Quick test: A point in time (2024, Monday, June) takes since. A length of time (three months, two days) takes for.

Reality: Many languages use one word for both. In English, they're not interchangeable.

17Present perfect vs. past simple

Use the past simple ("I finished") for a finished time. Use the present perfect ("I have finished") when no specific time is given and it connects to now.

Wrong	Right
I have published it last week.	I published it last week.
Did you ever write a newsletter?	Have you ever written a newsletter? (life experience)

Quick test: If you mention when it happened ("yesterday," "in 2023"), use the past simple.

Reality: American English is more relaxed about this than British English. Clarity matters more than the rule.

Part 4 · Mistakes 18–20

Punctuation

Small marks, big difference in meaning.

18Apostrophes in plurals

Apostrophes show possession ("the writer's desk") or missing letters ("don't"). They don't make words plural.

Wrong	Right
Download the PDF's.	Download the PDFs.
Music from the 1990's	Music from the 1990s
The writers desk (one writer)	The writer's desk
The writer's desks (many writers)	The writers' desks

Quick test: If you just mean "more than one," no apostrophe.

Reality: Most style guides now write decades and acronym plurals without apostrophes: 1990s, PDFs, APIs.

19Comma splices

A comma splice joins two complete sentences with only a comma. Commas follow sentence structure, not where you'd pause for breath.

Wrong	Right
The draft was ready, I sent it.	The draft was ready, so I sent it.
	The draft was ready. I sent it.
	The draft was ready; I sent it.

Quick test: If both halves could stand alone as sentences, a comma alone isn't enough. Add "and," "but," or "so," or use a period.

Reality: A period is almost always the simplest fix, and shorter sentences read better on phones anyway.

20That vs. which (and the commas)

In American English, that introduces information you need to identify something, with no commas. Which adds extra information, set off with commas.

Wrong	Right
My laptop, that is five years old, still works.	My laptop, which is five years old, still works.
The post, that went viral, was short.	The post that went viral was short. (one specific post)

Quick test: If you use commas, use which, never that.

Reality: British English often uses which without commas too. The comma rule is the part that matters everywhere.

Save this

Quick-test cheat sheet

Confusion	5-second test
its / it's	Can you say "it is"? Use it's.
your / you're	Can you say "you are"? Use you're.
their / there / they're	Ownership / place / "they are"
then / than	Comparing? Use than.
affect / effect	Action = affect, End result = effect
fewer / less	Can you count it? Use fewer.
who / whom	He = who, him = whom
e.g. / i.e.	Example given / in essence
I / me	Remove the other person and test
a / an	Listen to the first sound
since / for	Start point = since, length = for
that / which	Commas? Use which.

Wrapping up

Catch them before you hit send

Grammar tools help, and you should use them. But they don't understand what you meant, and AI tools can smooth your writing into something that's correct but no longer sounds like you.

The most reliable habit is simple: before you publish or send anything important, read it once slowly, ideally out loud. Most of the mistakes in this list jump out the moment you hear them.

And if English isn't your first language, be kind to yourself. Native speakers make every one of these mistakes too.

Proofread with fresh eyes

Paste your draft to check its length, sentence count, and reading level before you publish.

Try How Many Words

Frequently asked questions

What are the most common grammar mistakes in English?

The most common are mix-ups between sound-alike words such as its/it's, your/you're, their/there/they're, and then/than. Other frequent mistakes include affect/effect, fewer/less, I/me, comma splices, and apostrophes used for plurals.

How do I know when to use its or it's?

Use it's only when you can replace it with "it is" or "it has." Otherwise, use its, which shows possession, like "the app and its features."

Is it fewer or less?

Use fewer for things you can count, like "fewer words," and less for amounts you can't count, like "less time."

Do people still use "whom"?

Yes, mainly in formal writing and after prepositions, like "to whom." In everyday speech and casual writing, "who" is widely accepted.

Can grammar checkers and AI tools catch all these mistakes?

They catch many of them, but not all. They can miss errors where both words are spelled correctly, and sometimes suggest changes to sentences that were already right. Reading your text slowly, ideally out loud, is still the best final check.