Their and your are possessive forms used as modifiers before nouns. They basically mean ‘belonging to them’ and ‘belonging to you’, respectively: their problem, your idea. You’re is a contraction of ‘you are’: You’re doing fine. And there is a word functioning in various parts of speech, but basically referring to a place: Your umbrella is over there. You can remember their by noting that it looks like they, except the y is changed to i and an r is added at the end. If you keep this in mind you also won’t spell it thier. You can remember your by noting that other pronouns also have possessive forms that end in an r, such as her, our, their. Also note that no possessive pronoun contains an apostrophe.