"Your" and "You're"

"You're" is short for you are.
"Your" shows that something belongs to you or is related to you (e.g., your car, your father).

Some writers are confused by "you're" and "your". A mistake involving these constitutes a grammatical howler.

+ You're:
"You're" is a contraction of "you are". It has no other uses. This is a 100% rule. If you cannot expand it to "you are" in your sentence, then it is wrong.

Examples:
"The trouble with the rat race is that even if you win, you're still a rat." (Lily Tomlin)
(Expands to you are - correct)
"Doing nothing is very hard to do. You never know when you're finished." (Leslie Nielsen)
(Expands to you are - correct)
"Please ask Joan to post the parcel when you're in London."
(Expands to you are - correct)
WRONG - "You said that you couldn't believe you're ears."
(Does not expand to you are - should be your)


+ Your:
The word "your" sits before another word (usually a noun or a pronoun) to show that it belongs to "you" (e.g., your car, your arm), is of "you" (e.g., your picture, your photograph) or is related to "you" (e.g., your uncle). "Your" is a possessive adjective. (Other possessive adjectives are: my, your, his, her, its, our and their.)

Examples:
"Our expert will answer your questions about pensions and savings."
(questions belonging to you)
"Sarah doesn't look like your sister."
(related to you)


HOT TIP - Not in formal writing:
Do not use contractions (e.g., you're, isn't, can't, don't, it's) in formal writing. Always expand them. One benefit of doing this is that you will never make a mistake with you're or it's (two notorious grammar villains).

YOURS NOT YOUR'S
The word "yours" is known as an absolute possessive (others are "ours", "his" and "hers"). There are no apostrophes in any absolute possessives. This is another 100% rule.


TAKE THE TEST - Select the correct version:
John, you're / your not going to take Simon to watch you're / your sister sing in that show? You're / Your crazy. He'll be bored from start to finish.

http://www.grammar-monster.com/easily_confused/youre_your.htm