English has three of these that sound alike, and two of them cause most of the trouble. "There" and "their" get swapped constantly in email and chat, and unlike most spelling slips this one gets noticed — it is the error people quietly judge. The good news is that each word contains a smaller word that gives it away, and once you see that you stop having to think about it.
An adverb of place, or the word that starts a sentence about existence: "there is," "there are."
A possessive determiner. It marks something as belonging to a group of people already mentioned.
Quick way to remember
Look for the smaller word hiding inside. "There" contains "here" — both are about place. "Their" contains "heir" — and an heir inherits, which is possession. The third one, "they're," is just "they are" squeezed together.