Carnegie Mellon

Many writers ask "Is this correct?" and "How do I fix it?" Focus on the highest-payoff problems: homophones, possessives, agreement, punctuation, hyphens, and spacing. Below are short diagnostics, wrong/right pairs, and ready rewrites for work, school, and casual writing.

Run the quick checklist in Rewrite help to find the error fast, then use a rewrite pattern to fix the sentence immediately.

Quick answer: the traps to check first

When a sentence feels off, check in this order: 1) homophones/possessives (your/you're, their/there/they're, its/it's); 2) subject-verb agreement and pronoun number; 3) comma splices/run-ons; 4) apostrophe misuse vs plurals; 5) hyphenation and spacing that changes meaning.

If you can expand to "you are," use you're; otherwise use your.
Test it's by expanding to "it is" or "it has"; if that fits, use it's, otherwise its.
Match the verb to the main subject (ignore intervening prepositional phrases).
Fix comma splices with a period, semicolon, or comma + coordinating conjunction.
Use hyphens in compound adjectives before a noun and one space between words.

Core explanation: what actually goes wrong

Most recurring errors come from three sources: similar-sounding words (homophones), unclear sentence structure (agreement or misplaced modifiers), and fast typing (missing punctuation, wrong hyphens, stuck words).

Start by asking what you mean: possession, contraction, location, or agreement? That usually reveals the correct form.

Meaning errors → homophones/possessives (your vs you're, its vs it's).
Form errors → subject-verb agreement, pronoun number (everyone is singular).
Punctuation/spacing → comma splices, hyphenation, stuck words (alot).

Real usage: pick the form by context (work, school, casual)

Formal writing requires precision; casual writing accepts contractions and singular they. Copy the examples that match your tone.

Work:
Wrong: The teams' decision was unanimous and they're happy with it. →
Right: The team's decision was unanimous, and they're happy with it. (Treat the group as a single decision-maker.)
Work:
Wrong: Please advise if your able to join. →
Right: Please advise if you're able to join.
Work:
Wrong: We discussed the data shows an issue. →
Right: We discussed the data; it shows an issue. (Fixes a run-on; note how your field treats data.)
School:
Wrong: Its clear the hypothesis failed. →
Right: It's clear the hypothesis failed.
School:
Wrong: The class were excited about the field trip. →
Right: The class was excited about the field trip. (Treat collective nouns consistently.)
School:
Wrong: She studied all night she still failed. →
Right: She studied all night, but she still failed.
Casual:
Wrong: Your amazing! →
Right: You're amazing!
Casual:
Wrong: I'm gonna lay down. →
Right: I'm going to lie down. (lie = recline; lay needs an object)
Casual:
Wrong: Alot of people said so. →
Right: A lot of people said so.

Grammar essentials: homophones, possessives, and small-word traps

Before you change a word, test whether you mean a contraction or possession. Small substitution tests beat memorizing long rules.

your vs you're: If "you are" fits, use you're; otherwise use your.
their / there / they're: their = possession; there = place; they're = they are.
its vs it's: expand "it's" to "it is" or "it has" to test.
fewer vs less: fewer for countable items; less for bulk/amount.
affect vs effect: affect = verb (to influence); effect = noun (result).

Subject-verb agreement and pronoun number

Ignore intervening phrases (with, along with, as well as). The verb must agree with the main subject. Words like everyone, each, somebody take singular verbs.

Collective nouns can be singular or plural by meaning-pick one consistently for your context.

The list of errors is... (singular subject "list" → singular verb)
Everyone/each/neither + singular verb (Everyone is ready)
Data: use "the data are" in formal writing or "the data is" when treating data as a mass noun-follow your style guide.

Work - Wrong: The number of applicants are growing. →
Right: The number of applicants is growing.
School - Wrong: Everyone have the syllabus. →
Right: Everyone has the syllabus.
Work - Wrong: My group were chosen to present. →
Right: My group was chosen to present.

Try your own sentence

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

Punctuation, comma splices, and spacing

A comma splice joins two independent clauses with only a comma. Fix it by using a period, a semicolon, or adding a coordinating conjunction. Also watch apostrophes (not for plurals) and spacing errors (alot, double spaces).

Hyphens can change meaning in compound modifiers before a noun: use hyphens to prevent ambiguity (well-known author, full-time job). Do not hyphenate after the noun ("the author is well known").

Comma splice fixes: replace the comma with a period; use a semicolon; or add "and/but/so" after the comma.
Apostrophes: avoid apostrophes for plurals (apples, not apple's); possessive plurals take the apostrophe after s (students' essays).
Spacing: use one space after periods; avoid spaces before punctuation and inside contractions.

Work - Wrong: I attached the file, please confirm. → Work -
Right: I attached the file. Please confirm.
School - Wrong: The students essay's were late. → School -
Right: The students' essays were late.
Casual - Wrong: He said , that he'd come. → Casual -
Right: He said that he'd come.

Examples: focused wrong → right pairs you can copy

Casual - Wrong: Can I borrow you're pen? → Casual -
Right: Can I borrow your pen?
Work - Wrong: There going to reschedule the meeting. → Work -
Right: They're going to reschedule the meeting.
School - Wrong: The cat licked it's paws. → School -
Right: The cat licked its paws.
Work - Wrong: This will effect our timeline. → Work -
Right: This will affect our timeline.
School - Wrong: She studied all night she still failed the exam. → School -
Right: She studied all night, but she still failed the exam.
Casual - Wrong: I'm going to lay down for a bit. → Casual -
Right: I'm going to lie down for a bit.
Casual - Wrong: Alot of people RSVP'd early. → Casual -
Right: A lot of people RSVP'd early.
Work - Wrong: The data shows a clear trend. → Work -
Right: The data show a clear trend.
Casual - Wrong: I should of emailed you. → Casual -
Right: I should have emailed you.

Rewrite help: a repeatable diagnostic and ready rewrites

Diagnostic checklist: 1) Expand suspect contractions ("you're" → "you are"). 2) Replace possessive forms with "of the" to test. 3) Identify the main subject and verb. 4) Look for two independent clauses joined only by a comma.

If the sentence still feels clumsy, break it into two sentences or move modifiers closer to what they describe.

If you see your/you're - expand or paraphrase to choose the correct form.
If two clauses are glued by a comma - pick period, semicolon, or add "and/but/so".
If a compound adjective precedes a noun - consider a hyphen to remove ambiguity.

School - Rewrite:
Wrong: Because the results were surprising, we discussed them in class and the professor was excited. → Right
rewrite: We discussed the surprising results in class, and the professor was enthusiastic.
Work - Rewrite:
Wrong: I attached the file in the email, please confirm receipt. → Right
rewrite: I attached the file to this email. Please confirm receipt.
Work - Rewrite:
Wrong: The committee have decided to postpone the meeting but they didn't explain why. → Right
rewrite: The committee has decided to postpone the meeting, but it did not explain why.
Casual - Rewrite:
Wrong: He loves hiking, his brother prefers movies. → Right
rewrite: He loves hiking; his brother prefers movies.

Memory tricks and similar mistakes to watch for

Use quick tests that stick faster than long rules. Repeat them when editing and they become automatic.

"You're" = "you are" (try the expansion).
"It's" = "it is" or "it has" (if expansion works, use it's).
Fewer = countable; less = bulk (fewer apples vs less sugar).
Affect (verb) → action; Effect (noun) → result.
That = essential clause (no commas). Which = nonessential (use commas).

School - Similar: Wrong: He picked fewer sugar. →
Right: He picked less sugar.
Casual - Similar: Wrong: I should of known. →
Right: I should have known.
Similar: Wrong: The book, that I read, was long. →
Right: The book that I read was long.

FAQ

Is it 'your' or 'you're' before a noun?

Use your before a noun (your report). Use you're only when you mean "you are" (You're welcome to join). Test by expanding to "you are."

When should I use 'its' versus 'it's'?

It's is a contraction for "it is" or "it has." Its is the possessive. If "it is" fits the meaning, use it's; otherwise use its.

How do I fix a comma splice quickly?

Replace the comma with a period, use a semicolon, or add a coordinating conjunction (and, but, so) after the comma. Pick the option that best preserves flow.

Should I hyphenate compound adjectives?

Hyphenate compound adjectives before a noun (well-known author, full-time job). Do not hyphenate after the noun ("the author is well known").

Is 'data' singular or plural?

Formally, data is plural (the data are). In many modern contexts it's treated as a mass noun (the data is). Follow your field's or instructor's convention.

Stuck on a sentence?

Paste the sentence into a grammar tool that explains suggestions, then try these quick edits: expand contractions, test possessives, and apply the rewrite patterns above.

Practice by saving a few examples from the lists above and correcting them until the fixes feel automatic in real writing.