If there’s one thing that will get a grammar pedant’s goat, it will be mixing up there, their, and they’re. Here’s a simple guide to help you remember the difference:
There refers to a position, a place. It’s the opposite to ‘here’—note the similar spelling: the both end with ‘ere.’ So, if it’s not here, it must be there.
This one indicates possession—it belongs to them, it is theirs. Imagine the ‘i’ in their as a waving stickman to help you remember.
The apostrophe in they’re shows you there’s a letter missed, indicating that they’re really means ‘they are.’ Imagine that little wiggly thing as a tiny ‘a.’