your vs. you're
This is the mistake people apologise for most and repeat most. Not because the rule is hard — it is one of the simplest in English — but because the two words sound identical, and the part of your brain that picks words while you type works on sound before spelling. Autocorrect will not save you either. Both are real words, correctly spelled, so nothing gets underlined and nothing gets flagged. The only defence is a test you can run without thinking, and there is a good one.
your
A possessive determiner. It marks something as belonging to the person you are addressing.
- Your application arrived before the deadline.
- I borrowed your charger — it's on the desk.
- Please confirm your availability for Thursday.
- Your being late is becoming a problem. (Possessive before a gerund — "you're" would be ungrammatical here, though almost everyone says "you being late" in speech.)
you're
A contraction of "you are." The apostrophe stands in for the missing "a."
- You're the only person who replied.
- If you're finished, we can start.
- You're going to want to sit down for this.
- You're your own worst critic. (Both words in one sentence, which is the fastest way to feel the difference.)
Why your and you're get mixed up
Your and you're sound the same and are among the most common slips in writing, even for people who know the difference.
Your shows that something belongs to the person you are addressing: your email, your account, your idea. You're is a contraction of "you are" and nothing else: you're welcome, you're right, let me know when you're ready.
The test is to expand it. Read the sentence with "you are" in place of the word. If it still makes sense, write you're. If it doesn't ("is this you are laptop?"), write your. It is the same pattern as its and it's, and their and they're: the apostrophe marks missing letters, not ownership.
Common mistakes
Your welcome!
✓You're welcome!
This means "you are welcome", so the contraction: you're.
Is this you're laptop?
✓Is this your laptop?
The laptop belongs to the person, so the possessive: your.
Let me know when your ready to start.
✓Let me know when you're ready to start.
"When you are ready" makes sense, so you're.
Quick way to remember
Read the sentence with "you are" in place of the word. If it still makes sense, you want "you're." If it collapses — "you are application arrived" — you want "your." The test never fails, because "you're" is nothing more than "you are" with a letter removed.
Questions
Is "your's" a word?
No. The possessive is yours, with no apostrophe: "is this yours?" Like its and hers, possessive pronouns never take one.
Can a sentence end with "you're"?
No. Contracted forms like you're can't come at the very end of a sentence. It is "I know who you are", never "I know who you're".