1. Have you ever thought you were all prepared for something, but you realized you had forgotten to do something important?
Yes, I have.
2. What were you preparing for? > (past continuous)
I prepared my homework diary. = I was preparing for my diary homework.
3. What had you done?
I had done my homework.
4. What had you forgotten to do?
I had forgotten bring my diary! = forgotten to bring

Good job^^