The hardest homophones for my students to ever learn were there, their and they're. Here is a great power point mini lesson to explain the differences. This is then followed by a quiz, which can be done whole group, made into a paper/pencil assessment or loaded into a student response system, like Beyond Question or IRespond. I have used this in my classroom as an introduction and as a review - maybe you can use it for both! Tested with real students in the classroom of Heidi Raki.