Here are the corrected sentences:
1. Have you found a pearl before?
2. We haven't celebrated Tom's birthday yet.
3. Carrie has just finished her homework.
4. I've never met an explorer before.
5. Has Jason taken the bottles to the bottle bank yet?
6. They've just broken the window with their ball.
7. Have Tommy and his dad been caving before?
8. Everyone has already eaten the cake.