that's they're (their)

That's, they're, and their sound the same but serve different roles: that's = that is/that has, they're = they are, their = possessive. Use two quick checks-expand the contraction and look for ownership-to fix most errors fast.

Quick rules

That's = that is/that has. They're = they are. Their = belonging to them (no apostrophe).

Say the phrase out loud and expand it: if it becomes "that is" use that's; if it becomes "they are" use they're.
If the word directly modifies a noun (their + noun), it's possessive-write their.
For formal writing, use full forms ("that is," "they are") to avoid ambiguity.

Core explanation: how each word functions

That's is a contraction of "that is" or "that has." Example: That's the rule. (That is the rule.)

They're is a contraction of "they are." Example: They're ready. (They are ready.)

Their is a possessive adjective. Example: Their bags are on the floor.

that's → that + is/has (needs an apostrophe)
they're → they + are (needs an apostrophe)
their → shows possession and appears before a noun (no apostrophe)

Rewrite help: quick templates & three-step checklist

Three-step checklist: 1) Expand the word-"that is" or "they are." 2) If it shows ownership before a noun, choose their. 3) If you're unsure, rewrite to remove the contraction (e.g., "that is" / "they are" or recast the sentence).

Prefer full forms in formal text: "that is," "they are."
Use short rewrites to eliminate ambiguity while keeping tone.
Keep a few saved templates for emails and essays to paste when unsure.

Rewrite example 1 - Wrong: Their going to fix there car. Quick fix: They're going to fix their car.
Formal: The technicians will fix the car they own.
Rewrite example 2 - Wrong: Thats the only option we have. Quick fix: That's the only option we have.
Formal: That is the only option available.
Rewrite example 3 - Wrong: Are you sure their okay? Quick fix: Are you sure they're okay?
Formal: Are you certain they are okay?

Examples you can copy (wrong → right)

Common errors with straightforward corrections. Keep these patterns in mind and paste the correct forms when needed.

Wrong: Thats a great idea. -
Right: That's a great idea.
Wrong: Their going to be late. -
Right: They're going to be late.
Wrong: I like their's answer better. -
Right: I like their answer better.
Wrong: The students forgot it's books. -
Right: The students forgot their books.
Wrong: Theyre planning to come tomorrow. -
Right: They're planning to come tomorrow.
Wrong: Thats their problem now. -
Right: That's their problem now.
Wrong: Its their's to decide. -
Right: It's theirs to decide.
Wrong: Your late with they're homework. -
Right: You're late with their homework.
Wrong: Thats not what their thinking. -
Right: That's not what they're thinking.
Wrong: I heard their arriving at 6. -
Right: I heard they're arriving at 6.
Wrong: Theres no evidence their guilty. -
Right: There's no evidence they're guilty.

School examples

Wrong: Each student must hand in they're form by Monday. -
Right: Each student must hand in their form by Monday.
Wrong: Thats the thesis statement we need to develop. -
Right: That's the thesis statement we need to develop.
Wrong: Their handing in there assignments late. -
Right: They're handing in their assignments late.

Work examples

Wrong: Thats the agenda for todays meeting. -
Right: That's the agenda for today's meeting.
Wrong: Their presenting the Q2 results at 3 pm. -
Right: They're presenting the Q2 results at 3 p.m.
Wrong: Please review their attached deck before the call. -
Right: Please review the attached deck before the call. (Or: Please review their attached deck.)

Casual examples

Wrong: Thats so funny 😂 -
Right: That's so funny 😂
Wrong: I think their coming later. -
Right: I think they're coming later.
Wrong: Its their's to decide. -
Right: It's theirs to decide.

Reduce repeat mistakes faster

Combine habit checks (expand/possession) with reviewing corrected examples. Scan the sentence types you write most-email subjects, meeting notes, assignments-and save polished templates to reuse.

Memory tricks that actually work

Two micro-tests stop most slips: 1) Expand the word out loud; 2) Look for a following noun to spot possession.

If you can say "they are," use they're.
If the word sits before a noun and expresses ownership, use their.
If you can say "that is" or "that has," use that's; otherwise avoid the apostrophe.

Try your own sentence

Test the whole sentence rather than the isolated word-context usually makes the answer obvious.

Real usage & tone: when to avoid contractions

Contractions are natural in speech, email, blogs, and chat. For formal reports, academic papers, or legal text, prefer full forms-"that is," "they are"-to reduce ambiguity and match the tone.

Informal: contractions are fine-check meaning.
Formal: prefer full forms for clarity and professionalism.
Possessive their is neutral and correct across registers when showing ownership.

Spacing, apostrophes, and hyphenation pitfalls

Many errors come from typing and autocorrect: missing apostrophes, stray spaces, or odd punctuation. Check for these before assuming a grammatical misunderstanding.

Do not write "that 's" with a space; correct: that's.
Search for "thats" and "theyre"-autocorrect often removes apostrophes.
Use a consistent apostrophe character (straight or curly) in your editor.

Wrong: that 's the point -
Right: that's the point
Wrong: theyre -
Right: they're

Grammar notes & related pitfalls

Singular they/their is widely accepted: "Each student should bring their laptop." In formal contexts, check your style guide. The same expand-and-possession checks work for similar tricky pairs like its/it's and there/their/they're.

Singular their is acceptable in modern usage to avoid gendered pronouns.
its vs it's: expand to "it is" to test for it's; use its for possession.
There vs their vs they're: location vs possession vs "they are."

Wrong: Its going to rain; their bringing an umbrella. -
Right: It's going to rain; they're bringing an umbrella.
Wrong: Their late with its homework. -
Right: They're late with their homework.

Similar mistakes to watch for

After you fix that's/they're/their, check these often-linked errors:

its vs it's - expand to "it is" to decide.
your vs you're - expand to "you are" for you're; use your for possession.
there vs their vs they're - location / possession / "they are."

Wrong: Your late with they're homework. -
Right: You're late with their homework.

FAQ

When should I use that's vs they're vs their?

Use that's for "that is" or "that has," they're for "they are," and their for possession. Expand the word in your head; if it becomes a full phrase, use the contraction; if it shows ownership, use their.

Is singular "their" correct?

Yes. Singular they/their is standard in contemporary English to avoid gendered pronouns. Consult your style guide for strict formal contexts.

Will grammar checkers catch all these mistakes?

Most checkers flag common errors, but context can trick them. Combine a checker with the expansion and possession tests for best results.

Is it always wrong to write "thats" or "theyre" without an apostrophe?

In standard English, yes-contractions need an apostrophe. Missing apostrophes usually come from typing haste or autocorrect.

What's the fastest habit to avoid these mistakes?

Pause and run two checks: expand to "that is" or "they are," and see if the word indicates possession before a noun. That habit fixes most slips.

Want a quick check?

If you still doubt a sentence, paste it into a grammar checker and run a scan. Use the examples above as templates and rely on a checker for edge cases while you practice the expansion test.