Their, They're, There and Your, You're

These words sound the same because English pronunciation doesn't care about grammar, it just flattened three spellings into one sound. 'Their' is a possessive determiner: it sits right before a noun and shows that noun belongs to someone, as in 'their coats.' 'They're' is a contraction of 'they are,' squeezed together with an apostrophe standing in for the missing letter. 'There' is an adverb of place, pointing at a location, as in 'put it there,' or it's the filler subject in sentences like 'there are five exits.'

The test exploits the fact that your ear can't hear the difference, so you have to test each word by function. For 'their' versus 'they're,' try expanding the word to 'they are.' If the sentence still makes sense, you need the contraction. If expanding it breaks the sentence, and a noun follows instead, you need the possessive 'their.' 'There' never shows ownership, so a phrase like 'there house' is always wrong no matter how natural it sounds read aloud; ownership always needs 'their.'

The same logic runs the your/you're pair, just with one fewer option since there's no place-word version. 'Your' is possessive, sitting before a noun: 'your documents.' 'You're' expands to 'you are': 'you're sure' works because 'you are sure' works. Both pairs live under the same test move: whenever you hit a word that sounds like it could be three or two different things, mentally expand the contraction first and see if the sentence survives.