Grammar Check guide
Common grammar mistakes, with examples and fixes
The mistakes that turn up most are sound-alike words (their, there and they’re; its and it’s; your and you’re; then and than; to and too), “should of” for “should have”, “a” where “an” belongs, a verb that doesn’t agree with its subject, and a word typed twice. A grammar checker catches most of them. A few, such as “your welcome” and comma splices, still need you to read for them.
Sound-alike words
Most grammar mistakes aren’t about grammar at all. They happen because two words sound the same and your fingers type the one you use more often. Knowing the test for each pair is quicker than learning the rule.
| Pair | Quick test | Right |
|---|---|---|
| their / there / they’re | Can you say “they are”? Then it’s they’re. Does something belong to them? Their. A place or “there is”? There. | They’re late because their train stopped over there. |
| its / it’s | Can you say “it is” or “it has”? Then it’s it’s. Otherwise its, like his and hers, which have no apostrophe either. | It’s been a long week. The dog wagged its tail. |
| your / you’re | Can you say “you are”? Then you’re. | You’re welcome to bring your car. |
| then / than | Than compares; then is about time or order. | She is taller than her sister, and then she grew again. |
| to / too | Too means “also” or “more than enough”. | I went to the shop, but it was too busy. |
| affect / effect | Usually affect is the verb and effect is the thing that happens. | The rain will affect the match; it had a big effect on us. |
| lose / loose | Lose rhymes with “choose” and is the opposite of win or keep; loose is the opposite of tight. | Don’t lose your keys; this knot is loose. |
In our tests, Grammar Check underlined “Their going to the match”, “A user should check there work”, “They’re house is big”, “Its been a long week”, “The dog wagged it’s tail”, “You’re car is outside”, “taller then her sister”, “I am to tired”, “He went too the shop” and “a big affect on us”, each with the right word as a one-tap fix. It did not catch “Your welcome to join us”, “Your right about that” or “Loose the weight”: a rules-based checker can only flag a word when the words around it make the mistake certain, so some sentences slip through. Read those pairs yourself.
“Should of”, “could of”, “would of”
Said aloud, “should’ve” sounds exactly like “should of”, which is why the mistake is so common in writing. It is always have: should have, could have, would have, must have, might have.
- We should of booked a table.
- I must of left it on the train.
- We should have booked a table. (Or: we should’ve booked a table.)
- I must have left it on the train.
Grammar Check underlines should of, could of, would of and must of and offers “should have” and so on. Contracted and question forms such as “I’d of told you” or “Could you of done it?” got past it in our tests, so watch for those.
A or an?
The rule is about the sound of the next word, not its first letter. Use an before a vowel sound and a before a consonant sound.
| Wrong | Right | Why |
|---|---|---|
| a hour | an hour | The h is silent: it starts with an “ow” sound. |
| a honest mistake | an honest mistake | Silent h again. |
| an university | a university | It starts with a “you” sound, which is a consonant sound. |
| an useful tip | a useful tip | Same “you” sound. |
| a umbrella | an umbrella | A plain vowel sound. |
Grammar Check flagged every “wrong” example in that table. Abbreviations follow the same rule by sound: an MP (“em-pee”), a UFO (“you-eff-oh”).
Verbs that don’t agree with their subject
A singular subject takes a singular verb; a plural subject takes a plural one. With a pronoun right next to its verb, the mistake is easy to see and easy for a checker to catch:
- She have a car.
- He don’t like it.
- We was late.
- I has a question.
- She has a car.
- He doesn’t like it.
- We were late.
- I have a question.
Grammar Check catches all four. Agreement gets harder when other words sit between the subject and the verb, and that is where every checker, ours included, misses some:
- “Each of the students have a book.” The subject is each, which is singular: “Each of the students has a book.”
- “There is three apples on the table.” After there is or there are, the verb agrees with what follows: “There are three apples.”
- “The results was clear.” Find the real subject (results) and match it: “The results were clear.”
None of these three were underlined in our tests. When a sentence has a long subject, say just the subject and the verb together (“results were”) to hear whether they match.
Doubled words, one word or two, and set phrases
These are slips rather than gaps in knowledge, and they’re exactly what a checker is best at, because you’ll read straight past them in your own writing.
| Wrong | Right |
|---|---|
| The the meeting starts at ten. | The meeting starts at ten. |
| I have alot of work. | I have a lot of work. |
| I recieved your letter. | I received your letter. |
| We were suppose to be there. | We were supposed to be there. |
| He did it on accident. | He did it by accident. |
| We need to discuss about the plan. | We need to discuss the plan. |
| for all intensive purposes | for all intents and purposes |
| Irregardless of the cost | Regardless of the cost |
| He is more taller than his brother. | He is taller than his brother. |
It also points at “I think that that is fine” and asks whether the second that is needed. Sometimes it is, so this kind of underline is a question, not an order: tap Ignore if you meant it.
Capitals and spacing
- A sentence starts with a capital. “It rained all day. we stayed in.” Grammar Check underlines we.
- “I” is always a capital. “i went to London” is flagged.
- Days, months, countries and people’s names take capitals. “monday”, “january” and “england” are underlined as spellings, with the capitalised word as the fix. Seasons don’t: “in spring”, not “in Spring”.
- No space before a comma, one after. “Hello ,how are you?” is flagged, and so is a double space between words.
Mistakes a checker can miss
No grammar checker understands what you mean; it matches patterns. These came through our tests without an underline, so they’re worth learning to spot yourself:
| Mistake | Wrong | Right | Underlined? |
|---|---|---|---|
| your / you’re before an adjective | Your welcome. | You’re welcome. | No: read for it |
| Comma splice | We went to the beach, it was sunny. | We went to the beach. It was sunny. (Or: …beach, and it was sunny.) | No: read for it |
| Past tense | I seen it yesterday. | I saw it yesterday. | No: read for it |
| Pronoun case | Between you and I, it’s fine. | Between you and me, it’s fine. | No: read for it |
| Pronoun case | Me and him went to the shop. | He and I went to the shop. | No: read for it |
| let’s / lets | Lets meet at noon. | Let’s meet at noon. | No: read for it |
| Missing letter | Thanks for you help. | Thanks for your help. | No: read for it |
| Meaning flipped | I could care less. | I couldn’t care less. | No: read for it |
Two habits catch most of them. First, read it aloud: your ear notices “I seen it” long before your eye does. If reading aloud isn’t practical, let Read Aloud read it to you. Second, for a comma splice, ask whether each half could stand alone as a sentence. If both can, a comma isn’t enough: use a full stop, a semicolon or a joining word such as and, but or so.
How to check your writing, step by step
- Paste it in. Open Grammar Check and tap Paste and check. It runs on your own device, so even a private email or a cover letter never leaves it.
- Fix the red first. Spelling is red, grammar orange, punctuation purple and style blue. Spellings are the surest fixes, so clear them first.
- Tap each underline. It says what’s wrong in plain words; tap the fix, or Ignore if you meant it. Names it doesn’t know go in your dictionary with Add to dictionary.
- Go through the issues list. It shows every issue in order, so nothing is missed in a long piece.
- Read it once more, aloud. For the mistakes in the table above.
If you write in British, Australian, Canadian or Indian English, choose it in Settings first, so “colour” isn’t flagged as a mistake. Our guide to British and American spelling lists the differences.
Questions
What is the most common grammar mistake?
Mixing up words that sound the same: their, there and they’re; its and it’s; your and you’re. They aren’t gaps in knowledge so much as slips of the fingers, which is why a checker that underlines them as you type helps.
Is it “should of” or “should have”?
Always “should have” (or “should’ve”). “Should of” comes from how “should’ve” sounds aloud. The same goes for could have, would have and must have.
Is it “a hour” or “an hour”?
An hour. The choice between a and an depends on the sound of the next word: hour starts with a vowel sound because the h is silent. For the same reason it is “a university”, which starts with a “you” sound.
Can a grammar checker catch every mistake?
No. Rules-based checkers catch slips such as doubled words, wrong articles and should of reliably, but miss some mistakes that depend on meaning, such as “your welcome” or a comma splice. Read important writing aloud once before you send it.
Is there a free grammar checker that doesn’t upload my writing?
Yes. Grammar Check runs Harper, an open-source grammar checker, inside your browser, so your writing is checked on your own device and never uploaded. Every check and fix is free, with no account.