On The List Universe there is a post on 10 common English errors that is worth reading. The number one error is no surprise – there, they’re, their. Here are their tips – remember:
They’re: The apostrophe is used here to replace a missing letter – the letter ‘a’. “They’re” means “they are” – it only mean “they are”, and can never mean anything else. So if you want to say that someone is happy, you say “they’re happy”. Remember, the apostrophe stands for a missing letter.
Their: This means “belongs to them” – it only means “belong to them” and nothing else. The confusion that has arisen over this word is no doubt related to the fact the an apostrophe is often used to denote possession – such as “John’s dog” – but when we are talking about “them” possessing something, we don’t use the apostrophe.
There: Everything else falls in to this category. “There is a happy man”, “Over there!”, “There aren’t many people at the party”.
Here is a little tip for remembering:
Their – “Their” has “heir” in it – an heir ultimately possesses items left to them in a will.
There – “There” has “here” in it – this can remind you that it refers to a place.