Some writers are confused by you're and your. A mistake involving these constitutes a grammatical howler. 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)  You said that you couldn't believe you're ears.  (Does not expand to you are – should be yourThe 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, 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") NOT IN FORMAL WRITING As a general rule, 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.   See also: What are adjectives? What are nouns? What are pronouns? What are absolute possessives? What possessive adjectives? List of easily confused words   (edited for errors while typing from a phone)