their / there / they’re
The possessive, the place word, and the contraction.
Their is the possessive: their keys. There marks a place, or stands in the expletive there is: Put it there. There is a crate. They’re is they are.
Examples
UseTheir keys are on the hook.
UsePut the crate there.
UseThey’re ready.
AvoidThere keys are on the hook.