Here are the correct forms to fill in the blanks:

1. If you **had had** something to eat before, you wouldn't feel sick now.
2. If Sandy **did not eat** so much junk food every day, she could have been in better shape.
3. If I **had known** her address, I could have visited her yesterday.
4. If he had a watch, he **would have been** at school on time last Monday morning.