What is the difference between their, there and they're?

Three words that sound identical and mean entirely different things. Each has one job.

Their is possessive. It shows something belongs to a group. Their car, their idea, their fault.

There indicates a place, or introduces a sentence. Put it over there. There are four options.

They're is a contraction of they are. They're late.

The tests are quick. For they're, expand it: if they are fits, the apostrophe version is right. For there, look for the word here hiding inside it — both are about location. For their, look for the heir inside it, and an heir inherits, which is possession.

The reason this trips people up is not ignorance of the rule. It is that all three are extremely common, and the brain selects words by sound when writing quickly. Most people who make this error know the difference perfectly well and would catch it on a second read. That makes it a proofreading problem rather than a grammar problem, which is why it survives in the writing of otherwise careful people.

Spellcheck will not help. All three are correctly spelled words, so nothing is flagged. Grammar checkers catch some instances but miss plenty.

One combination worth watching: there's means there is or there has, never theirs. And theirs is possessive with no apostrophe, like hers and ours.

Reading a draft aloud catches these more reliably than reading silently, because the errors are invisible to the eye and obvious to the ear once you slow down.

One more pairing worth separating: there's versus theirs. There's is a contraction of there is. Theirs is possessive and takes no apostrophe, like hers and ours. There is no such word as their's.