Use they're as a contraction for they are. Usually, if you can replace they're with they are in the same sentence, and it still makes sense, then you are using it correctly.
"They're coming over for dinner."
"I wonder who they're talking to."
Download our free worksheet now! You can download and print out this free There, They're, Their Worksheet to help you or someone you study the difference between there, they're, and their. (See answers)