1. All the students love the principal. He is very kind. (because of)
All the students love the principal because of his kindness.
2. I have forgotten the tickets. I’m sorry for it. (having)
I'm sorry for having forgotten the ticket.
3. I saw it with my own eyes. I believed it. (if)
If i hadn't seen it with my own eyes,i wouldn't have believed it.
4. The house at the end of the street has been sold. It has been empty for years. (which)
The house at the end of the stree which has been empty for years has been sold.
5. It's a pity he doesn't take the doctor's advice. (wish)
I wish he took the doctor's advice.
Good Luck !