Hey kids! Welcome to Learning with Fridgey, a series where I'll be looking at common grammar mistakes. Yeah, talk about exciting! Keep in mind that as a (sigh) 'limey' I use British English spellings (which basically means, amongst other things, you'll find extra u's in various places such as armour, favour, etc). Don't worry, I know that not everyone can talk proper like what I do [sic].
#1 - They're there in their room
One of the most common mistakes I see when reading forum comments and instant messages is the incorrect use of there/their/they're. I'll set out the groundrules of how we determine which to choose with some examples.
They're - They're is a contraction of the words 'they are', and is only ever used in this fashion. For example, "They're not very good at running", "They're more stupid than they look", "And they're off!"
Their - Their is possessive, and is used when you are talking about something that is owned by or has a relationship to the people about whom you are talking. For example, "Their 360 red ringed", "Their mother was an idiot", "I tried to steal their shoes".
There - There has the most wide range of uses, and is used in all other instances not described above. It is used when talking about a location of something/someone ("Dave's over there"), used to introduce sentences (such as There are many reasons why Britain sucks ass"), to describe a point in an action "He paused there for effect", to call attention to something "There they are!". There are other uses, but bear in mind - if you're not trying to say 'they are' or talking about something or someone belonging to the people you're speaking about, it's always 'there'.