land lover (landlubber)

These mistakes jump out in quick proofreading: their/they're/there, your/you're, its/it's, misplaced apostrophes, and writing 'could of' instead of 'could have'. Below are short rules, fast tests, and many wrong/right pairs you can copy into emails, essays, or texts.

Start with the quick rules, then scan the focused sections and ready-to-use rewrites.

Quick answer: short rules

Their = possession; they're = they are; there = place or dummy subject. Your = possession; you're = you are. Its (no apostrophe) = possession; it's = it is / it has. Use could have (could've), not could of. Apostrophes mark possession or contractions, not plurals.

Test contractions by expanding them (they're → they are; you're → you are).
Test possession by asking "whose?" (their, your, its).
Don't use apostrophes for plurals (1990s, apples).

Core problems: five traps and quick fixes

Use two quick checks: expand suspected contractions; ask whether the word shows ownership. Below are concentrated wrong/right pairs, grouped so you can copy or scan quickly.

Wrong: Their late to the meeting and left their bag there.
Right: They're late to the meeting and left their bag there.
Wrong: Your the only one who can fix this problem.
Right: You're the only one who can fix this problem.
Wrong: Its raining, the dog hurt it's paw.
Right: It's raining; the dog hurt its paw.
Wrong: I could of finished if I had the files.
Right: I could have finished if I had the files.
Wrong: The report have John'ss errors.
Right: The report has John's errors.

Their / They're / There - quick diagnostic and examples

If you can replace the word with "they are," use they're. If it answers "whose?" use their. Otherwise use there for places or sentences starting with "There is/are."

Test: substitute "they are" for contraction; ask "whose?" for possession.

Work - Wrong: Their submitting the budget later.
Right: They're submitting the budget later.
Work - Right: Please put the files on their desk over there.
School - Wrong: There dogs refused to cooperate.
Right: Their dogs refused to cooperate.
School - Right: They're presenting the poster at 2 p.m.
Casual - Wrong: Is their a party tonight?
Right: Is there a party tonight?
Casual - Right: They're on their way to the cafe over there.

Your vs You're - the one-line test and examples

Substitute "you are." If the sentence still makes sense, use you're; otherwise use your for possession.

Test: replace with "you are" to check for you're.
Watch common slips: "Your welcome" (wrong) vs. "You're welcome" (right).

Work - Right: Your presentation was great today. (possession)
Casual - Wrong: Your going to love this book.
Right: You're going to love this book.
School - Usage: Make sure your name is on the cover page.

Its vs It's and other apostrophe pitfalls

It's is a contraction for it is / it has. Its (no apostrophe) shows possession. Apostrophes do not form plurals.

Names ending in -s are style-dependent (James's vs James'); follow the relevant style guide.

Test: expand it's → it is / it has; if that fails, use its.
Never use an apostrophe to make a plural (three dogs, 1990s).

Wrong: Its been a long day for the team.
Right: It's been a long day for the team.
Work - Right: The lab updated its equipment schedule; it's available to book now.
School - Right: The student submitted its assignment on time. (for an organization or object)
Casual - Right: It's my turn to pick the movie; its cover looks cool.
Wrong: I have three dog's.
Right: I have three dogs.

Could have / would have / should have - why "could of" is wrong

Could've sounds like could of in speech, but 'of' is incorrect. Use could have or could've (same for would/should).

Test: replace 'of' with 'have'-if have fits, write have.
Correct forms: could have (could've), would have (would've), should have (should've).

Wrong: I should of told you earlier.
Right: I should have told you earlier. (or I should've told you earlier.)
Work - Rewrite: Bad: "I could of completed the audit." → Better: "I could have completed the audit."
Casual - Rewrite: Bad: "I would of come if I knew." → Better: "I would have come if I had known."
School - Right: If I'd studied more, I would have passed.

Try your own sentence

Test the whole sentence rather than a single word-the surrounding words usually make the correct form clear.

Hyphens, spacing and small punctuation rules

Hyphenate compound modifiers before a noun: small-business owner. After the noun, omit the hyphen: the owner is a small business owner.

Use one space after a period. Avoid underscores in prose; prefer hyphens or spaces.

Compound modifier before noun = hyphen (state-of-the-art equipment); after = usually no hyphen.
One space after a period in modern style.
Apostrophes mark possession or contractions only; don't use them for plurals or emphasis.

Wrong: She is a small business-owner who travels frequently.
Right: She is a small-business owner who travels frequently.
Wrong: The username was posted as land_lover in the article.
Right: The username was posted as land-lover or Land Lover in the article.
Wrong: We need to re-open the study.
Right: We need to reopen the study. (check the dictionary for closed compounds)

Rewrite help: a 4-step checklist with live rewrites

Checklist: 1) Read aloud and expand contractions. 2) Test possession vs contraction. 3) Check apostrophes and plurals. 4) Simplify or split long sentences.

Below are quick rewrites you can paste directly.

Work - Original: "Your expected to prepare the summary by Monday."
Rewrite: "You're expected to prepare the summary by Monday."
Work - Original: "Their responsible for the vendor list over there."
Rewrite: "They're responsible for the vendor list over there."
School - Original: "Its clear the experiment failed because of bad controls."
Rewrite: "It's clear the experiment failed because of poor controls."
School - Original: "Whose presenting the poster?"
Rewrite: "Who's presenting the poster?"
Casual - Original: "I could of met you at the cafe."
Rewrite: "I could have met you at the cafe."
Casual - Original: "Your the best singer I've heard."
Rewrite: "You're the best singer I've heard."

Memory tricks: mnemonics that stick

Short cues you can use when writing fast.

Their = has "heir" (ownership). If it shows belonging, pick their.
They're = expand to "they are." If that works, use they're.
Your = possession; you're = you are. Say "you are" to test you're.
Its (no apostrophe) = possessive like his/her; it's = it is or it has.
Could of? Think "have" - of is never right here.

Similar mistakes to watch for (brief fixes and examples)

After the core five, these cause frequent errors. Learn the simple test for each.

then vs than - then = time; than = comparison.
affect vs effect - affect = usually a verb; effect = usually a noun.
lie vs lay - lie = recline (no object); lay = put (needs object).
fewer vs less - fewer with countable nouns; less with mass nouns.
who vs whom - who = subject; whom = object (try he/him replacement).

Wrong: She had less apples than me.
Right: She had fewer apples than I did.
Wrong: Who's job is this?
Right: Whose job is this?
Wrong: He did good on the test.
Right: He did well on the test.

FAQ

Is it your or you're?

Use you're when you can replace it with "you are." If the word shows ownership, use your.

When should I use its vs it's?

Use it's only as a contraction (it is / it has). Use its (no apostrophe) to show possession.

Can I use contractions in formal writing?

Some formal contexts prefer full forms. When in doubt, follow your organization or instructor's style guide; otherwise use full forms in formal reports and papers.

Why do people write 'could of'?

Because could've sounds like could of when spoken. In writing, replace 'of' with 'have'-the correct forms are could have or could've.

How can I stop making these mistakes?

Use the 4-step checklist: read aloud, expand contractions, check apostrophes, and simplify sentences. Practice with the example rewrites and use a second-pass checker when needed.

Try fixing one sentence now

Pick a sentence you wrote, apply the 4-step checklist, and rewrite it. Small, frequent corrections build better habits.